[Bridge]

(A radar screen is beeping. A man in a mostly blue uniform goes to a younger man in mostly red.)
VORSHAK: What do you think?
BULIC: Too small to be a hunter-killer.
VORSHAK: Could be one of their probes trying to locate our position. Computer scan.

[Silurian cruiser]

(The sleek craft settles onto an underwater crater.)
SCIBUS: No hostile movements registered.
ICHTAR: Excellent, Scibus, but continue to monitor their activities. We must remain undetected until we are ready to act.

[Bridge]

(A woman in mostly grey reports.)
KARINA: We've lost it, Commander.
VORSHAK: Readings?
KARINA: It seems to be organic in structure, and radiating heat.
(Another woman, in mostly brown, speaks.)
PRESTON: Check.
VORSHAK: Volcanic debris?
(A second man in blue-grey joins in.)
NILSON: More than possible. We're close to the oceanic fault here.
(The screen continues to pan around looking for the little craft.)

[Tardis]

(The Doctor has had a little haircut since the last adventure. Turlough enters from the interior.)
TURLOUGH: How are we doing?
DOCTOR: Oh, on target, it seems. You changed your mind about going home.
TURLOUGH: Well, I, er, I thought I would learn more if I stayed with you. It's true.
DOCTOR: Of course.
TURLOUGH: I mean it.
DOCTOR: I believe you. I'm just a little doubtful about how resolute you'll remain.
TURLOUGH: Time will tell.
DOCTOR: Yes, indeed. Aboard the Tardis it always does.
TURLOUGH: Where are we going?
DOCTOR: Earth.
TURLOUGH: What for?
DOCTOR: I promised to show Tegan a little of her planet's future. We're almost there. You could let her know.
(Turlough leaves and the time rotor stops unexpectedly. The Doctor flicks switches, concerned.)

[Bridge]

VORSHAK: Nothing. What's the matter?
BULIC: I think we should launch a reconnaissance probe.
VORSHAK: Forever cautious.
BULIC: I've spent too long on a Sea Base not to be. And given how unstable the current political situation is, an attack is not to be unexpected.
VORSHAK: Launch an unmanned probe.
KARINA: Sir.
VORSHAK: Happier?
BULIC: Thank you, sir.
VORSHAK: The base is to remain on full alert, and Maddox?
MADDOX: Sir?
VORSHAK: If there is enemy activity outside, we could go to missile run, so stand by.
(The young man looks very worried.)
MADDOX: Yes, sir.
(Moments later, Maddox leaves his post and goes into the next compartment.)

[Silurian cruiser]

SCIBUS: The base has launched a probe.
ICHTAR: The Myrka can deal with it.
TARPOK: We are ready to begin, Ichtar.
ICHTAR: Good. For hundreds of years our Sea Devil brothers have lain entombed, waiting patiently for this day. It concerns me that they did not wake up as we had planned in the first place.
(Ichtar and Tarpok walk slowly through a rock-cut passageway.)
ICHTAR: Their enforced period of hibernation may have caused much muscular and organic degeneration. We shall soon see, Tarpok. Proceed.
(Tarpok presses some squidgy buttons, and through a glass window we see the liquid level dropping in a large area.)

[Computer room]

(Karina joins the worried Maddox.)
KARINA: What's the matter?
MADDOX: I can't do it.
KARINA: Of course you can.
MADDOX: You saw me out there.
KARINA: You wouldn't be here if there was any doubt about your fitness for the job.
MADDOX: I'm a student on attachment to Sea Base Four to study an experienced sync operator in action, not to take his place. I'm not ready.
KARINA: Until Lieutenant Michaels' replacement arrives, there isn't anyone else.
MADDOX: I know. I'd feel much happier if there'd been a proper investigation into the Lieutenant's death.
KARINA: It was a careless accident. There was nothing to investigate.
MADDOX: Lieutenant Michaels was careful to the point of paranoia. Men like that aren't electrocuted carrying out simple maintenance.
KARINA: You reported your suspicion?
MADDOX: Of course, but Commander Vorshak's not interested. He just keeps going on about what a marvellous opportunity this is for me to gain hard experience.
KARINA: The Commander's right.
MADDOX: Maybe he is, but I'm also aware that if we so go to missile alert I don't think I can press that button.
KARINA: Listen to me. Don't throw your whole career away because you don't feel quite ready. That's silly. Lieutenant Michaels' replacement arrives the day after tomorrow. There may not be a missile run before then anyway.
MADDOX: All right.
KARINA: Come back to the bridge?

[Tardis]

TURLOUGH: Now what?
DOCTOR: It's my own fault. I should, er, I should have changed it for a Type 57 while I had the chance.
TEGAN: We're where we should be, aren't we?
DOCTOR: Oh, yes. Very close to Earth, in orbit above the atmosphere belt.
TEGAN: So what's the problem?
DOCTOR: Er, slight hiccup in our time zones. We're too far advanced. I'm sorry.
TURLOUGH: Doctor, look.
(The scanner shows five bright dots flying in formation. They resolve themselves into a single object.)
TEGAN: What is it?
DOCTOR: It's a robot weapons system. It seems to be scanning us.
SENTINEL [OC]: This is Sentinel Six. You have entered a forbidden military zone. Transmit your security clearance code.
DOCTOR: Reset the coil cut-out.
(Turlough obeys.)
DOCTOR: Sentinel Six, could you repeat your instructions, please.
SENTINEL [OC]: This is Sentinel Six. You have entered a forbidden military zone. Transmit your security clearance code or you will be destroyed.
DOCTOR: Sentinel Six, we have no hostile intentions. Our presence here is purely temporary. All we need is a bit of time to alter coordinates.
TEGAN: Why doesn't it answer?
DOCTOR: Well, it's obviously thinking it over.

[Psycho-surgical unit]

(A woman doctor in white is checking a computer bank when Nilson enters, carrying a blue case.)
NILSON: It's time to move, Solow. We've got our man.
SOLOW: Maddox?
NILSON: Yes, you were right. He is psychologically unsuited for his work.
SOLOW: I'm glad to hear it. I must admit I was a little concerned lately that my diagnosis was inaccurate.
NILSON: No, you can congratulate yourself and stop worrying. The sudden demise of Lieutenant Michaels has paid us an immediate dividend.
SOLOW: You're a hard man, Nilson, but you forget, I'm a doctor. Murder does not come easily to a person of my training.
NILSON: Don't bleat, Solow. We've waited long enough for an opportunity like this.
SOLOW: I realise that.
NILSON: And nothing must go wrong. So if your conscience bothers you, lock it away in a strong box until our task is completed.

[Tardis]

(The image of Sentinel Six fills the scanner.)
TEGAN: Please hurry, Doctor. That thing isn't going to lie there contemplating its navel forever.
DOCTOR: Don't panic, Tegan. Now look, just push this. Almost there.
SENTINEL [OC]: This is Sentinel Six. You are formally identified as a hostile intruder.
DOCTOR: Sentinel Six, we are unarmed.
SENTINEL [OC]: Repeat. Transmit your security clearance codes. This is your final warning.
DOCTOR: Sentinel Six, we need just a little more time then we will be on our way.
TEGAN: Doctor, look!
(Sentinel Six fires on the Tardis. It lurches and the light goes red.)
DOCTOR: We're out of control.
TURLOUGH: We're going to crash!
DOCTOR: Not if I can perform a materialisation flip flop.
(The time rotor moves and the lights return to normal.)
DOCTOR: Stage one.

[Bridge]

(Beep, beep, beep.)
VORSHAK: What is it?
KARINA: The reconnaissance probe's been destroyed, sir.
VORSHAK: Perimeter defence stand by. Feed the coordinates of the hostile directly to them. We'll blast it out of the water.
KARINA: I can't. The only thing registering on the scanners is marine life.
PRESTON: That can't be so. A creature strong enough to destroy the probe doesn't exist.
VORSHAK: Well?
BULIC: Karina's right.
(The screen lights up. Missile Run Green Alert.)
VORSHAK: Maddox, verify.
MADDOX: Computer has started countdown.
VORSHAK: Your assessment, Bulic?
BULIC: It's difficult to tell. It could be a practice run ordered by the computer. Equally it could have been triggered by what's happened out there.
VORSHAK: Then we must assume the missile run is for real. All teams to battle stations.
PRESTON: Battle teams one, two and three take up defence position.
KARINA: Sentinel Six reports engaging unidentified flying object. Attempts to shoot it down proved unsuccessful, but it has now disappeared.
(Missile Run Yellow Alert)
BULIC: Missile computer is on automatic targeting. Arming of the proton missiles is in progress.
(The lights turn red.)
VORSHAK: Prepare to sync up. Maddox! I said take up your position. The function of the base depends on you! What's wrong?
MADDOX: I can't do it, sir.
VORSHAK: Without you, our missiles are useless.
MADDOX: Don't you think I realise that?
VORSHAK: Sync up, Maddox. Find out what the computer is doing. Come on, we could be at war!
(Reluctantly, Maddox goes over to a chair under a tall column.)

[Tardis]

(The Tardis materialises somewhere inside the Sea Base.)
TURLOUGH: We made it.
TEGAN: I don't believe it.
TURLOUGH: I don't think the Doctor does either.
(The Doctor comes out from underneath the console.)
DOCTOR: Yes, well, I admit it was close.

[Bridge]

(Maddox has a jack plug socket in the side of his head.)
BULIC: Relax. Just access what the computer tells you and leave the final decision to the Commander.
MADDOX: But I still have to press the button.
BULIC: It may not come to that.
(A dome comes down over Maddox's head and plugs him in. It hurts.)
BULIC: We have sync up to missile computer, Commander.
VORSHAK: Go ahead, Maddox.
(Maddox starts tapping at a small keyboard in front of him.)
MADDOX: Missiles are locked onto targets.
(Red Alert)

[Tardis]

TEGAN: Where are we?
DOCTOR: Still in the same time zone.
TURLOUGH: And on Earth?
DOCTOR: I think so. Let's find out.

[Bridge]

MADDOX: Missiles armed.
VORSHAK: Prepare to fire.
(Maddox puts his left hand over a red button.)
BULIC: Countdown to missile launch, one hundred and fifty and counting.

[Outside the Tardis]

TEGAN: It's a bit chilly in here.
TURLOUGH: It seems to be a ship of some kind.
DOCTOR: Or submarine. There's no motion. It could be on the seabed. Come on.
(He leads them up a spiral staircase.)

[Bridge]

BULIC: Thirty seconds to launch.
VORSHAK: Stand by.
(The word Simulation comes up on the screen. Vorshak breathes a deep sigh of relief.)
VORSHAK: We can breathe again.
(Maddox is released from the sync, and collapses.)
NILSON: Commander!
VORSHAK: Get him out of here.
NILSON: Take him to the PS unit.
VORSHAK: What a time for a practice run.
BULIC: You realise the base is defenceless with Maddox in the state he is.
PRESTON: And we still haven't established what destroyed our probe, nor what Sentinel Six shot at.
(A guard carries Maddox out over his shoulder.)
VORSHAK: Sound the all clear, but the base it to remain on full alert.

[Sea Base]

TEGAN: What's that noise?
DOCTOR: Yes, of course. A Sea Base.
(The Doctor tries to rub dirt off a sign on a bulkhead door.)
TURLOUGH: Doctor.
(Turlough offers a handkerchief.)
DOCTOR: Oh, thank you.
(Chemical store.)
DOCTOR: Ah ha. That, Turlough, was the all clear.
TEGAN: Must be some kind of research station.
DOCTOR: No, no, it's a Sea Base. A rather special kind of undersea military colony. Help me get this open, would you? Armed with the sort of missiles that kill life but leave everything else intact.
(The Doctor and Turlough push at the door, but it refuses to move. Tegan gently slides it to one side and goes through.)
TURLOUGH: Proton?
DOCTOR: Very probably.

[Silurian cruiser - outside the hibernation chamber]

TARPOK: The Sea Base has completed a practice missile run.
ICHTAR: Then our presence remains undetected. Continue to monitor the Sea Base. This process will not take long. Have we any indications of the conditions within?
SCIBUS: No. The temperature level inside is below the range of the sensors.

[Bridge]

VORSHAK: Launch another probe.
KARINA: Sir.
(The intercom beeps.)
VORSHAK: Bridge.
NILSON [OC]: Nilson here. We have a problem with Maddox. I think you should come down, Commander.
VORSHAK: Right. Bulic, you're in command. I'll be in the psycho-surgical unit.
BULIC: Sir.

[Chemical store]

TEGAN: What year are we in?
DOCTOR: Around 2084.
TEGAN: Little seems to have changed since my time.
DOCTOR: Absolutely nothing, Tegan. There are still two power blocs, fingers poised to annihilate each other. Hexachromite.
TURLOUGH: What do they use that for?
DOCTOR: Hexachromite gas. It's part of a sealing compound for undersea structures. It's lethal to marine and reptile life. I thought they would have banned it by now.
TEGAN: Progress doesn't seem to have solved anything.
DOCTOR: No.

[Psycho-surgical unit]

(Maddox is unconscious on a diagnosis couch.)
NILSON: He's suffering from severe stress. It's our considered opinion that he's unfit for duty.
VORSHAK: But I need him. Without a sync operator, this base will cease to have any military function.
NILSON: And I realise that, otherwise I'd hesitate proposing the one option that you still have.
VORSHAK: And that is?
NILSON: Release the duplicate programme disc. That way we could probe deeper into Maddox's mind, break through the barriers and reassure him.
VORSHAK: But I can't do that, not without authority from Sea Base Command.
NILSON: If we are to maintain operational efficiency, there really is no other way.
SOLOW: I will take full responsibility. There are humanitarian as well as military considerations here. Maddox will have a total mental collapse unless treated.
VORSHAK: All right.
(Vorshak goes to a nearby bench and places his hand on a scanner pad. Then he punches in a code and a portion of the bench rises. Another palmprint reader and code pad are there.)
VORSHAK: But don't let this disc out of your sight.
(A plastic screen slides down and he removes one small disc out of a row of six.)
VORSHAK: If it gets into enemy hands
NILSON: We understand, Commander.
SOLOW: I will return it to you immediately we're finished with it.

[Sea Base]

TEGAN: Do you know where we're going?
DOCTOR: Er, yes, yes, the bridge. Repairs to the Tardis will take some time. Better they're done with the Captain's permission.

[Psycho-surgical unit]

(Maddox has two jack-plug sockets, one either side of his head, it seems. Solow plugs a unit into it and rolls it forward so that it surrounds his head.)
SOLOW: Ready.
(Nilson inserts the disc and starts it running.)
NILSON: You're sure this will work?
SOLOW: The information on that disc is fed straight into the deepest control centres of his brain. Whatever commands we give him, Maddox has no other choice but to obey.

[Sea Base]

TURLOUGH: Up or down?
DOCTOR: Well, if I remember rightly, the command centre's at the top of the structure.
TURLOUGH: Right.
DOCTOR: No, wait!
(Too late. Turlough has summoned the lift.)
TURLOUGH: What's wrong?
DOCTOR: This.
TURLOUGH: I pressed for the lift.
DOCTOR: Yes, it has to be done in sequence. It's what's called security. We must find someone in authority and quickly. Come on.
(They start to run down the passageway and a bulkhead slides across in front of them.)
DOCTOR: Security screen.
TEGAN: What do we do now?
DOCTOR: Well, they've sealed off the area. Back to the Tardis. We'll be safer there.

[Bridge]

PRESTON: Perimeter defence is now complete.
VORSHAK: Where are they?
PRESTON: Area C.
VORSHAK: Double the guard round the PS unit.
PRESTON: Battle teams eight and ten, urgent command.
VORSHAK: Katina, tell Solow and Nilson what has happened. Any sign of an intruder, they must destroy the disc. What do you think?
BULIC: First that unidentified object, then the destruction of the probe, now this. I don't believe in that much coincidence.
VORSHAK: Agreed. We must act fast. You and Preston get the combat teams moving. Whoever's in that area, find them.
BULIC: Preston.

[Psycho-surgical unit]

(The disc is still running, then Nilson stops it. Solow unplugs Maddox and they sit him up.)
NILSON: How do you feel, Maddox?
MADDOX: Fine, Controller.
NILSON: Good.

[Chemical store]

DOCTOR: This way.
PRESTON [OC]: Let's check the Chemical store.
(The Doctor, Turlough and Tegan hide just before Preston enters with armed troops.)
PRESTON: Nothing. Come on.
(She leaves through the Fire Exit.)
DOCTOR: They're between us and the Tardis. Come on, back this way.

[Silurian cruiser- outside the hibernation chamber]

SCIBUS: We are ready to enter.
ICHTAR: It has been a long time. Much could have gone wrong.
(He opens the door to the hibernation chamber, where serried ranks of Sea Devils are kneeling in dry ice.)

[Reactor chamber]

(The Tardis crew enter through a door marked No Unauthorised Access onto a catwalk around a multilevel area.)
TEGAN: Why have we come in here?
DOCTOR: You didn't read what it said outside?
TEGAN: That's why I'm asking. I don't like running into chambers that have Radiation, Keep Out written on the door.
TURLOUGH: What are you going to do?
DOCTOR: Try and start a diversion.
(He looks at various control panels.)

[Silurian cruiser- outside the hibernation chamber]

ICHTAR: Revive the warriors.

[Reactor chamber]

DOCTOR: That's it.
TURLOUGH: What have you done?
DOCTOR: I've hotted things up a bit. I've set the reactor on overload.
TEGAN: It'll explode!
DOCTOR: Oh, I shouldn't think so. They've got a couple of hours to sort out what I've done.
TEGAN: This is madness!
TURLOUGH: The Doctor's right. It will tie up dozens of their people.
DOCTOR: And in the confusion, we can slip back to the Tardis.
(Bulic and a guard enter.)
BULIC: No. We can't risk a shot in here.
DOCTOR: When I say run, run. Run!
(Tegan and Turlough run.)
BULIC: Get them!
(The Doctor stands and raises his arms. As the guard walks past him, he hits him.)
DOCTOR: I'm so sorry.
(The Doctor elbows a second guard and runs, but the first guard grabs his ankle.)
TEGAN: Doctor!
(The Doctor kicks himself free, but the second guard grapples him, ending up with pushing him over the railing into the water below. Tegan grabs a set of chains hanging nearby.)
TURLOUGH: Tegan, no!
(The Doctor floats to the surface, face down.)
TEGAN: Doctor!
TURLOUGH: There's nothing we can do.
TEGAN: We can't just leave him.
TURLOUGH: Face it, Tegan. He's drowned.
(The Doctor sinks below the surface again.)

Part Two

[Reactor chamber]

(Tegan and Turlough run for the far door.)
BULIC: After them!
(Meanwhile, in the reactor cooling pool, the Doctor gets a handy lungful of air from an oxygenating vent, then swims over to a hatchway and tries to open it.)

[Outside the Reactor chamber]

(Turlough sabotages the door locking mechanism.)
TURLOUGH: Right, that should slow them down a bit.
TEGAN: We should have tried to help him.
TURLOUGH: We couldn't! Come on, let's get back to the Tardis.

[Reactor chamber]

BULIC: Go round and get this door open.
GUARD: Sir.
(Bulic uses a small communicator.)
BULIC: Commander, this is Bulic. I'm in the reactor.
(The Doctor has made it into a small airlock.)

[Silurian cruiser hibernation chamber]

SCIBUS: The warriors have survived.
(The Sea Devils begin to move.)

[Psycho-surgical unit]

(The Doctor carefully opens the other hatch to the airlock and looks out.
Nilson answers his communicator.)
NILSON: Nilson.
VORSHAK [OC]: The three invaders have attacked the reactor in area E. I need you on the bridge at once.
NILSON: I'm on my way, Commander.
SOLOW: Nilson, who are the invaders?
NILSON: I don't know, but with Vorshak distracted, it may be possible to activate Maddox sooner than we expected.

[Silurian cruiser hibernation chamber]

SCIBUS: There will be a short period of orientation.
ICHTAR: Excellent. When all is ready, I will instruct Sauvix and his Sea Devil warriors on the plan of attack.
(The Sea Devils start to stand up and move around.)

[Sea Base]

(The guard sent to open the sabotaged door gets a nasty electric shock. Elsewhere, guards come round a corner in front of Tegan and Turlough.)
TURLOUGH: Run!
(Tegan gets past the bulkhead before it closes on Turlough.)
TURLOUGH: Go, Tegan! Save yourself!
(Meanwhile, the Doctor comes across the fallen guard. He checks his pulse.)
DOCTOR: You'll live.
(He takes off his wet jacket and pullover.)

[Silurian cruiser - outside the hibernation chamber]

(The Sea Devil leader comes out of the hibernation chamber.)
SAUVIX: Sauvix, Commander of Elite Group One.
ICHTAR: Ichtar, sole survivor of the Silurian Triad, and my companions Scibus and Tarpok. We welcome the revival of our blood-related comrades.
SAUVIX: We are yours to command.
ICHTAR: All is prepared.

[Outside the Reactor room]

(The Doctor has stripped the guard, tied him up, and put on his protective clothes. He sniffs.)
DOCTOR: What have you been eating?
(He runs away as a pair of guards walk across a junction and totally fail to notice their fallen comrade.)

[Bridge]

NILSON: Have you found the intruders?
VORSHAK: One of them. He's being brought up now. I'd like you to be present at the interrogation.
NILSON: How did they breach our security?
VORSHAK: That's what I want you to find out.

[Sea Base]

(Tegan runs along. An arm grabs her, and she squeals, then sees who it is.)
TEGAN: Doctor, I thought you were dead.
DOCTOR: Yeah, so did I for a moment. Where's Turlough?
TEGAN: The guards caught him. We've got to help him.
DOCTOR: Yes, all right. First I must get you back to the Tardis.
TEGAN: I want to help find Turlough.
DOCTOR: Yes, all right.
TEGAN: What have you been eating?
DOCTOR: Come along, Tegan.

[Silurian cruiser]

(More Sea Devils file out of the hibernation chamber.)
ICHTAR: How soon will your warriors be ready for combat?
SAUVIX: Battle orientation automatically commenced the instant we were revived.
ICHTAR: Excellent, Sauvix.
TARPOK: Your plan of attack.
ICHTAR: Study it well, for the ape primitive base must be taken intact.
(Sauvix leaves with the cylinder.)

[Bridge]

VORSHAK: Bridge.
GUARD [OC]: The reactor's been stabilised, sir. We now have full power again.
VORSHAK: Thank you.
(Turlough is escorted in by Bulic and two guards.)
GUARD: Sir.
BULIC: This is Commander Vorshak, the senior officer on this base.
TURLOUGH: I've told
VORSHAK: Now listen, and listen carefully. You have a choice. Cooperate, tell us all you know, and you'll be treated honourably.
TURLOUGH: I am
VORSHAK: But should you remain stubborn, it'll be a long and painful business, so start talking.
TURLOUGH: I've told him, and now I'm trying to tell you. We are not enemy agents, saboteurs
NILSON: Then why were you attempting to destroy the reactor?
TURLOUGH: If the Doctor had intended to destroy it, it would be lying in pieces at your feet.

[Outside the bridge]

DOCTOR: Good. Now, you must stay outside, Tegan.
TEGAN: Why?
DOCTOR: Shush.
TEGAN: What in the world can I hope to achieve out here?
DOCTOR: My peace of mind. Please, Tegan, don't argue. Now, I don't need to tell you what to do should anything go wrong.
TEGAN: No, Doctor. Good luck, Doctor.
DOCTOR: Yes, thank you.
(The Doctor is about to open the door when a man comes out. He dodges aside then enters just before the doors close.)

[Bridge]

(Everyone is paying attention to Turlough, and don't notice the Doctor quietly enter and move around the outside of the area.)
TURLOUGH: The Tardis, it, it's a kind of ship. I know it doesn't seem to make much sense, but that's how we got here.
NILSON: He's insulting our intelligence, trying to convince us he's nothing more than a lost tourist.
BULIC: If you'd nothing to hide, why didn't you declare yourselves to us immediately.
TURLOUGH: We were trying to.
VORSHAK: What do you think?
NILSON: He may be telling the truth, though I doubt it. But we can't be sure without delving deeper into his mind.
VORSHAK: Then see to it, Nilson.
NILSON: Take him to the PS unit.
(The Doctor intervenes.)
TURLOUGH: Doctor!
(The Doctor turns his gun on Bulic, who is going for a weapon.)
DOCTOR: I wouldn't, if I were you. Well, gentlemen, it seems we have a problem.

[Tardis]

(Preston and her team have found the Tardis. They go inside.)
PRESTON: Good heavens. Look, there's another door. Search through there.
(The guards go into the interior while Preston looks at the console.)

[Bridge]

BULIC: Your move, Doctor.
DOCTOR: So it seems. Perhaps it's time for a little mutual trust.
(The Doctor hands his weapon to Vorshak.)
DOCTOR: Turlough was telling the truth.
(The intercom beeps.)
VORSHAK: Bridge.
PRESTON [OC]: We've found the intruder's craft. It's amazing. It isn't of this planet.
VORSHAK: Is it armed?
PRESTON [OC]: Not that we can tell.
VORSHAK: Well, leave a guard on it. Return to the bridge.
DOCTOR: Well, Commander?
VORSHAK: Well, it seems the boy was telling the truth after all.

[Silurian cruiser]

SAUVIX: The warriors are armed and at their stations.
ICHTAR: Excellent, Sauvix. It is time to begin.
(The sleek craft takes off.)

[Outside the bridge]

(Preston and a guard walk up to where Tegan is hiding.)
PRESTON: You want something?

[Bridge]

VORSHAK: All right, I believe you're not hostile.
DOCTOR: Thank you.
VORSHAK: But that doesn't mean I trust you. I should like to see a demonstration of your ship.
DOCTOR: When I've repaired her.
(Preston enters with Tegan.)
PRESTON: Commander, the third member of the crew. I found her lurking outside.
TURLOUGH: Are you all right?
KARINA: Commander, the screen.
(The Silurian vessel is on visual.)
KARINA: It's heading towards the base.
VORSHAK: Is that the thing we picked up before?
KARINA: Yes, sir.
MADDOX: We're getting a clear sensor scan. It is a ship of some kind, but definitely not one of ours.
VORSHAK: Prepare to fire energy tracers.
DOCTOR: No, don't!
VORSHAK: You recognise it?
DOCTOR: Yes. You mustn't attack.
VORSHAK: You're hardly in a position to give orders, Doctor.
BULIC: Ready to fire, Commander.
DOCTOR: Do so and you'll regret it. I know what that ship is.
VORSHAK: You're telling us not to defend ourselves?
DOCTOR: I'm telling you you have no defence. That's a Silurian battle cruiser.
PRESTON: Silurian?
DOCTOR: The race that ruled this planet long before your species evolved.
BULIC: Do we fire, sir?
DOCTOR: Trust me. You must try and make contact with it, find out what they want.
VORSHAK: It's not what they want, it's what I want, Doctor. And I want to keep them away. Open fire.
(Bulic obeys. Energy beams head towards the vessel then the screen flares white.)

[Silurian cruiser]

TARPOK: The deflector is locked on to their energy beam. Their external weapons system is now suppressed.
ICHTAR: Proceed as planned.

[Bridge]

VORSHAK: Fire again. Fire again!
BULIC: Energy systems are dead.
VORSHAK: You knew that would happen.
DOCTOR: I did try and warn you.
PRESTON: What have they done?
DOCTOR: It's a particle suppressor. They turned your energy beam back on you. They could have blown this base apart. They certainly have enough reason to.

[Silurian cruiser]

(The battle cruiser settles on the sea bed a short way from the Sea Base.)
TARPOK: We are in position.
ICHTAR: Release the Myrka. Sauvix, proceed to your station. When the Myrka begins its work, you and your warriors commence the attack on the main entry point.

[Bridge]

VORSHAK: I want damage reports as soon as possible. Maddox, set the computer to analyse the Silurian weapon.
MADDOX: Sir.
TEGAN: When did you meet the Silurians, Doctor?
DOCTOR: A long time ago. I let them down then, it seems I'll do so again.
TEGAN: Are they hostile?
DOCTOR: They're honourable. All they ever wanted to do was live in peace.
(Guards patrolling the main airlock area notice the outer door has been activated.)
MADDOX: It doesn't compute, sir.
DOCTOR: Commander.
VORSHAK: Yes, what is it?
DOCTOR: You can't fight them.
VORSHAK: I can try.
DOCTOR: Inform your people. Let them know what's going on down here.
VORSHAK: I can't do that. I must maintain radio silence. I can't risk revealing the Sea Base's position to the enemy.
DOCTOR: Friend or enemy, it's a distinction that's lost on the Silurians, I assure you. To them you're all the same. Ape-descended primitives. An evolutionary error they obviously mean to correct.
(The intercom beeps.)
VORSHAK: Bridge.
GUARD [OC]: Commander, there's something outside airlock one. It's trying to force the outer door.
VORSHAK: Stay there. (to Bulic) Get a team down there. Preston, deploy the duty guards to cover all the other airlocks.
PRESTON: Yes, Commander.
DOCTOR: Let me go with them. I know the Silurians. If I can talk to them, we may be able to avoid bloodshed.
VORSHAK: You can try, but remember we have no reason to trust you. Watch him. The first sign of treachery, kill him.

[Airlock]

PRESTON: Any sign of entry?
GUARD: No, ma'am.

[Airlock One]

BULIC: Any change?
GUARD: No, sir.
(The airlock indicator lights go out.)
BULIC: The magnetic locks on the outer door are blown. They're in the airlock.
TEGAN: How can you tell?
BULIC: The automatic pumps have started up.

[Bridge]

VORSHAK: Bridge.
PRESTON [OC]: Airlock five is also under attack, Commander.
VORSHAK: I'll come down at once. Karina, put the reserve team on combat alert. Maddox, stand by for sync up. We may have to contact Sea Base Command after all.
NILSON: But Commander, if we do
VORSHAK: The Doctor may be right. If he is, these creatures are a threat to all mankind.
(Solow enters and Vorshak leaves.)
SOLOW: Nilson, we must speak.
(They go to the back of the bridge.)
SOLOW: The base is on war alert.
NILSON: It's all right. While Vorshak is busy, I'm in command of the bridge.
SOLOW: Are we going to activate Maddox?
NILSON: Yes.
SOLOW: What about the Silurians? What if they manage to break in?
NILSON: We shall neutralise the Sea Base whatever happens, and with the help of Maddox we'll destroy all vital circuitry to do with missiles and communications.

[Airlock One]

(The inner hatch is beginning to buckle.)
BULIC: Take up defensive positions.
TURLOUGH: Will it hold?
BULIC: I wouldn't count on it.

[Bridge]

(Nilson presses a button on a small control pad. Maddox stands up and sways.)
KARINA: Are you all right? Maddox. Maddox.
NILSON: Doctor Solow will take care of Maddox.
KARINA: But he's needed here.
NILSON: I'm afraid in his present state he's useless.
KARINA: Shall I inform the Commander?
NILSON: No, no, no, no. I'll do that. You just return to your duties. I'm sure Doctor Solow will have Maddox back at his terminal as soon as possible.
(Solow helps Maddox walk away.)

[Airlock One]

TEGAN: What if these Silurians don't want to listen, Doctor?
DOCTOR: Look on the bright side, Tegan.
(The hatch begins to gives way, revealing something of the creature on the other side.)
DOCTOR: Oh dear.
TEGAN: What is it?
(The creature is a massive, multi-legged relative of the Silurians and the Sea Devils, and it is very hostile.)
DOCTOR: The Myrka.

[Airlock Five]

VORSHAK: Well?
PRESTON: Commander, they're inside the airlock.
VORSHAK: That didn't take long.
(Someone starts to burn their way in.)

[Computer room]

(Karina enters to see Maddox busy working on a computer panel.)
KARINA: What are you doing here?
SOLOW: His duty, as I dictate it.
KARINA: Sabotage. Maddox, stop it! Maddox, can you hear me? Stop it!
(Nilson uses his control box. Maddox turns and grabs Karina by the throat.)
KARINA: No!
NILSON: Now kill her, Maddox.

[Airlock One]

BULIC: Take aim.
DOCTOR: I'm afraid the Myrka takes quite a lot to impress.
BULIC: Fire!
(After few rounds.)
BULIC: We hit it.
(The Myrka ducks below the mangled doors. Tegan moves forward.)
TEGAN: He must have killed it.
DOCTOR: Tegan!
(The Myrka smashes the doors down completely, hitting Tegan. The Doctor goes to try and lift it off her legs.)
BULIC: Fire again.
DOCTOR: Are you all right?
TEGAN: How would you feel if you'd had a door fall on you?
DOCTOR: Anything damaged?
TEGAN: My dignity.
DOCTOR: Turlough, help me!
TURLOUGH: It's no good, we can't lift it.
(The Myrka towers over them.)
DOCTOR: Keep back. Don't let it touch you.
(The Myrka hits a guard, killing him with an energy bolt.)
DOCTOR: Bulic, get your people out of here!
BULIC: Clear the airlock!
DOCTOR: You too, Turlough.
TURLOUGH: But Tegan
DOCTOR: Get out!
TEGAN: You go as well, Doctor.
DOCTOR: I'm not leaving you.
TEGAN: There's no point in us both dying.
DOCTOR: Who's talking about dying?

[Outside Airlock One]

(The Sea Base personnel retreat. Bulic reports by communicator.)
BULIC: The creature's in, sir, and our weapons are useless against it.
VORSHAK [OC]: Use grenades.
BULIC: The Doctor and the girl are trapped in the airlock.
VORSHAK [OC]: Has the creature passed the bulkhead door?
BULIC: No, sir.
VORSHAK [OC]: Then close it! Seal the bulkhead off immediately.
TURLOUGH: No!
VORSHAK [OC]: What do you mean, no? Do it! The safety of the Base depends on it.
(Bulic goes to the control panel.)
TURLOUGH: No, wait.
(A guard gets Turlough off Bulic and up against the bulkhead. Bulic closes the door.)

[Airlock One]

DOCTOR: One last try.
(The Myrka plants one of its forefeet on the edge of the airlock door and the other end springs up, freeing Tegan.)
DOCTOR: Thank you so much.
TEGAN: They've sealed us up!
DOCTOR: Brave heart, Tegan.
TEGAN: Brave heart? That thing's going to kill us.

Part Three

[Outside Airlock One]

TURLOUGH: Open it!
BULIC: The controls are locked. It can now only be opened by the bridge.
(Turlough gets the rifle off the guard who is trying to restrain him, and punches him in the stomach.)
BULIC: Let him go.
(Turlough runs off. Bulic hands another guard's weapon to the stricken man.)
BULIC: Stay on guard, Sergeant.
GUARD: Yes, sir.
BULIC: I must see the Commander. The rest of you follow me.

[Airlock One]

(The Doctor is fiddling with the weapon.)
TEGAN: That won't do much good.
DOCTOR: Get ready. Mind your eyes, Tegan.
(The Doctor throws the weapon's ammunition pack at the Myrka. Big flash! The Doctor tries the door lock.)
TEGAN: What did you do to it?
DOCTOR: Simple. The charge from the Myrka's body blew up the magazine.
TEGAN: It's blinded.
DOCTOR: Yes, temporarily.

[Bridge]

(Turlough bursts in.)
TURLOUGH: The master control to bulkhead one, where is it?
NILSON: The Commander's orders were to keep that bulkhead closed.
TURLOUGH: I know what the Commander's orders were, but now I'm giving you mine. Open that bulkhead.
(Turlough points the weapon at Nilson.
The bulkhead opens as the Myrka gets its sight back. The Doctor and Tegan get out and the Sergeant enters. The Myrka zaps him then jams a forelimb into the closing bulkhead door.)
NILSON: The hydraulic valve has blown.
TURLOUGH: The bulkhead opened?
NILSON: Yes. But I'm not so sure it closed.
(Turlough runs out.)

[Sea Base]

TEGAN: What do we do if the Myrka breaks in, Doctor?
DOCTOR: Well, hopefully we can stop it before it does too much damage.
(Guards pass them, running towards Airlock One.)
TEGAN: You said yourself it's practically indestructible.
DOCTOR: Yes, we all have an Achilles' heel, Tegan, including the Myrka. Now we must hurry. I must speak to Vorshak.

[Bridge]

(Solow enters cautiously.)
SOLOW: Nilson, I have hidden the woman's body. The escape pod is ready for us. We can leave as soon as Maddox has completed his work.
NILSON: Good.
(The Myrka is loose on the Base. It electrocutes some personnel who are pressed up against a bulkhead by touching the metal with its tail.

[Silurian Base]

TARPOK: The Myrka has broken through the bulkhead door.
ICHTAR: Command it to make for its objective.

[Airlock Five]

(Vorshak's communicator beeps.)
VORSHAK: Vorshak.
NILSON [OC]: The creature's broken into the Base. Turlough forced us to open the bulkhead door.
VORSHAK: Sound full alert, Nilson. (to Bulic) Get your best people to Airlock One immediately. They must stop that creature.
BULIC: Our weapons have no effect on it.
VORSHAK: Have them try!
DOCTOR: He's right, Commander. Your weapons are useless against it.
VORSHAK: But not against you. You know what Turlough has done?
TEGAN: Yes, he saved our lives.
VORSHAK: I ordered that bulkhead to be kept closed.
DOCTOR: The bulkhead wouldn't have kept the Myrka out for long.
PRESTON: Commander, the seals are about to blow.
DOCTOR: If it's any consolation, I may just know how to stop it.
VORSHAK: When I want your help, I'll ask for it.
DOCTOR: You've got nothing to lose.
VORSHAK: I should have you shot.
DOCTOR: After I've dealt with the Myrka.
VORSHAK: All right, but I can't spare you any help.
DOCTOR: I need just one person. Someone in authority who can get things done.
VORSHAK: Very well. Preston, go with him.
PRESTON: Sir.
VORSHAK: Kill him if he gives you any trouble. That's an order. The rest of you, get ready.
(Preston leads the Doctor and the limping Tegan off at a trot.
Vorshak watches as the bulkhead doors open to reveal a troop of Sea Devils. The two sides start shooting at each other immediately. A guard falls.)
VORSHAK: Get back! Back.
(The humans take their fallen comrade with them and move back outside the airlock area. Turlough hears the weapons fire.)

[Outside Airlock Five]

BULIC: They've got us outgunned, Commander.
VORSHAK: I know. It all depends now on whether this bulkhead can hold them.

[Airlock Five]

SAUVIX: Bring forth the cutting device.

[Outside Airlock Five]

TURLOUGH: The Doctor and Tegan, where are they?
VORSHAK: They're safe, and contributing to the defence of this Base, as you are.
TURLOUGH: Sorry?
VORSHAK: You're volunteering your services to defend this bulkhead.

[Sea Base]

(Preston's communicator beeps.)
PRESTON: Preston.
GUARD [OC]: The Myrka's in corridor seven.
PRESTON: Delay it's progress as much as possible.
GUARD [OC]: We'll do our best.
DOCTOR: It can only be heading for one place.
PRESTON: The bridge?
DOCTOR: Yes, the bridge.
PRESTON: I'll inform the Commander.
DOCTOR: Do you have ultraviolet converters on the Base?
PRESTON: Yes.
DOCTOR: Good. Now, will the Myrka have to pass this way to get to the bridge?
PRESTON: If it maintains its present course.
DOCTOR: Perfect. I need a converter brought here.
PRESTON: Tech unit.

[Outside Airlock Five]

(Turlough checks the state of the bulkhead where the Sea Devils are burning through.)
BULIC: It's not going to hold.
VORSHAK: We must break radio silence.
BULIC: Have every enemy listening post pinpoint our position?
VORSHAK: Sea Base Command has to know about these creatures. We have no choice.

[Sea Base]

(A piece of equipment with a metre-wide dish is rolled up.)
DOCTOR: Ah. Set it up here.
PRESTON: That'll be all. Carry on.
(The guards leave.)
DOCTOR: Now, what do you think. Will the lighting circuit bear maximum converter load?
PRESTON: Just about.
TEGAN: What have you got in mind?
DOCTOR: I'm planning to bring a little sunshine into the Myrka's life.

[Bridge]

BULIC [OC]: The Commander is on his way up to signal Sea Base Command. Have Maddox stand by for sync up.
NILSON: Understood.
SOLOW: What are you going to do?
NILSON: Maddox must finish what he has started.

[Sea Base]

PRESTON: I suppose you know what you're doing.
DOCTOR: The Myrka is a creature of the inkiest depths, or it was until the Silurians tinkered with its biology. Anyway, it has little tolerance to light and hopefully none at all to ultraviolet rays.
TEGAN: Can you be sure?
DOCTOR: No, Tegan. Perhaps you should ask it nicely to go away.

[Computer room]

NILSON: The work goes well. Wait in the escape pod. I'll join you when I can.
SOLOW: What about Maddox's conditioning disc?
NILSON: Take it with you. It'll provide essential knowledge.
SOLOW: Good luck.

[Sea Base]

PRESTON: Ready?
TEGAN: Ready now?
DOCTOR: Yes, almost.
(Solow comes around the corner.)
PRESTON: Doctor Solow.
(Solow hurries away. The Doctor points away from the UV converter.)
DOCTOR: Look!
(And switches on. It works. He switches it off again.)
DOCTOR: Perfect. Almost.
(A little way away, Solow dodges a pair of guards then walks straight into the path of the Myrka. Foolishly, she tries to fight it unarmed, touches it and dies. The disc lies on the floor by her hand, where the guards find it when they return.)
GUARD: 43Y.

[Airlock Five]

(The Silurians enter, carrying a box.)
ICHTAR: How fares the course of battle?
SAUVIX: The outcome is certain. The ape primitives are no match for my warriors. They will be crushed.

[Sea Base]

VORSHAK: You have everything you need?
DOCTOR: Yes, I think so.
TEGAN: What happened to Turlough?
VORSHAK: He's defending Airlock Five with Bulic. Doctor, I've decided to take your advice. I'm contacting Sea Base Command.
DOCTOR: Very wise.
(The guards rush up.)
GUARD: The Myrka's coming this way, sir.
(He hands the disc to Vorshak.)
GUARD: It killed Doctor Solow. We found this by the body.
DOCTOR: Something wrong?
VORSHAK: I pray not, Doctor. Get down to Airlock Five. Preston, come with me.
(Vorshak's communicator beeps.)
VORSHAK: Vorshak.
BULIC [OC]: They're in, sir. The Sea Devils are everywhere.
VORSHAK: You must hold them.

[Outside Airlock Five]

BULIC: We'll try, Commander.
VORSHAK [OC]: Do what you can.
BULIC: Back! Get back!
(Turlough scores a direct hit on a breast plate. It has no effect.)

[Computer room]

(Nilson uses his control pad and Maddox jerks.)
NILSON: Don't die on me, Maddox. Not yet. Not till you've served your purpose.
(Maddox continues to rearrange printed circuits in a large computer bank.)
NILSON: Now, hurry.

[Outside Airlock Five]

ICHTAR: Are the ape primitives in full retreat? Do you control all strategic areas?
SAUVIX: Yes, Ichtar.
ICHTAR: Excellent. We expect the Myrka to take the bridge very soon.
SAUVIX: Then the outcome is doubly certain.

[Bridge]

(Vorshak shows Nilson the disc the guard found by Doctor Solow.)
NILSON: Yes?
VORSHAK: Maddox's conditioning disc. You were ordered to return it to me.
NILSON: Solow was in charge of that disc. I assumed she had.
VORSHAK: Where is Maddox?
NILSON: Inside, checking the sync circuit.
VORSHAK: Get him in here.
(Preston goes to the rear door while Vorshak holds Nilson's shoulder. She sees what Maddox is doing.)
PRESTON: Commander.

[Computer room]

VORSHAK: Maddox! What are you doing?
(Maddox throws Vorshak to the floor. Unseen, Nilson uses his control pad. Finally, Maddox collapses.)
PRESTON: He's still alive.
VORSHAK: Check the damage to the other computers.

[Sea Base]

DOCTOR: Here it comes. Now we'll know if my theory was right.
TEGAN: Switch it on.
DOCTOR: Just a few more feet.
TEGAN: Switch it on!
DOCTOR: All right, Tegan, close your eyes. Make a wish!
(The UV light hits the Myrka hard. It collapses and the Doctor switches off. The Myrka stops twitching.)
TEGAN: Is it dead?
DOCTOR: Very. Let's get back to the bridge.
(In another corridor.)
TARPOK: The Myrka has been destroyed.
ICHTAR: The ape primitives have more cunning than we thought. Sauvix, divert your warriors. We must capture the bridge without delay.
SAUVIX: At once, Ichtar.

[Computer room]

VORSHAK: You were in charge of the bridge. How could you not know what was going on in here?
PRESTON: Commander! Karina's dead. She's been murdered.
VORSHAK: You'll answer to a court martial for this, I promise, but first, you'll revive Maddox and decondition him. I want him ready for sync up. We must signal Sea Base Command.
NILSON: I'm afraid we can't do that, Commander.
VORSHAK: You'll do it!
(Nilson produces a small hand gun.)
NILSON: Your weapons, please.

[Outside the bridge]

TEGAN: What do the Silurians want, Doctor?
DOCTOR: Obvious. Control the Base and the proton missiles
TEGAN: And you control Earth?
DOCTOR: Or destroy it.

[Bridge]

VORSHAK: I trusted you, Nilson.
NILSON: Don't take it so hard, Commander. Like you, we were only doing our duty.
DOCTOR: The Myrka is dead, Commander.
NILSON: That's far enough, Doctor.
VORSHAK: It seems Nilson is an enemy agent.
TEGAN: For the Silurians?
VORSHAK: No, our human enemies.
NILSON: The power bloc opposed to this Sea Base.
DOCTOR: There isn't time for your petty feuds.
NILSON: I know what I'm doing, Doctor.
DOCTOR: Do you? Before long, the Silurians and the Sea Devils will have control of this Base.
VORSHAK: And control of the proton missiles.
NILSON: Missiles they can't fire. Maddox has irreversibly rigged the computers to that effect.
DOCTOR: The technology of these creatures predates yours by millions of years. If they intend to fire those missiles, they have the means, I assure you.
NILSON: They won't have time. As soon as I've left in the escape pod, this Base will be attacked and everything on it, including the creatures and all of you, will perish.
(A pale-faced Maddox staggers in behind Nilson.)
MADDOX: Nilson, you made me kill Karina. Now you're going to
(Maddox raises a weapon, and Nilson uses the control pad without looking.)
NILSON: I'm sorry, Maddox, but your usefulness is at an end.
(Maddox falls to the floor. Preston rushes to him.)
NILSON: Leave him! He's dead.
(The Doctor makes a grab for Nilson's weapon and gets elbowed aside. Nilson takes Tegan hostage.)
NILSON: If any of you try to follow me, she dies.

[Sea Base]

(Turlough and Bulic get trapped between two groups of Sea Devils.)
SAUVIX: Isolate them.

[Bridge]

PRESTON: The computers have been badly damaged.
(Vorshak's communicator beeps.)
VORSHAK: Vorshak.
GUARD [OC]: There's not much more we can do, sir. The Sea Devils have just breached the bridge perimeter.
VORSHAK: Where are Bulic and Turlough?
GUARD [OC]: Dead, or taken, Commander.
VORSHAK: I'm sorry.
DOCTOR: I'm going after Nilson.

[Sea Base]

SAUVIX: The way to the bridge is cleared.
(The Silurians walk on.)

[Crew barracks]

(Turlough watches a pair of Sea Devils patrol outside.)
TURLOUGH: We've got to get out of here.
BULIC: To go where? The Sea Devils are all over the place.
TURLOUGH: The Tardis? At least we'd be safe there.

[Sea Base]

(Tegan is struggling in Nilson's arms as they reach the dead Myrka.)
NILSON: Be still, woman. Pity all the Doctor's efforts were to no avail.
TEGAN: You haven't got away yet.
(Tegan makes Nilson fall over the Myrka as the Doctor runs up.)
NILSON: Stay where you are. You were very foolish to follow me, Doctor.
DOCTOR: Let Tegan go. She's of no use to you now. Killing us won't make your escape any easier. You fire, and every Sea Devil in the area will come running.
NILSON: I'll take that risk.
DOCTOR: Tegan, make a wish!
(Tegan covers her eyes.)
NILSON: Goodbye, Doctor.
DOCTOR: Goodbye.
(The Doctor switches on the UV converter with Nilson straight in the path of its beam. Tegan gets away from him, then the converter blows up. Sea Devils arrive to find a blinded Nilson flailing around, so they kill him. Then they see the Doctor and Tegan.)
SAUVIX: Your turn.

Part Four

[Sea Base]

DOCTOR: How do you do? I'm the Doctor. Haven't we met before?
(Sauvix gestures for the Doctor and Tegan to walk ahead of him.)

[Crew barracks]

(Turlough points to a grill high up in a bulkhead.)
TURLOUGH: What's this?
BULIC: A ventilation shaft.

[Bridge]

(The Doctor and Tegan are brought in at gunpoint.)
TARPOK: The Sea Devil warriors have captured the reactor room.
ICHTAR: Excellent.
VORSHAK: Allow my crew to surrender.
ICHTAR: It is they who insist upon fighting.
(Scibus enters from the computer room.)
SCIBUS: The damage is assessed, Ichtar. The computer can be restored to normal functioning.
ICHTAR: Perfect.
DOCTOR: Ichtar.
TEGAN: You know him?
DOCTOR: Yes. I thought he'd been killed.
ICHTAR: Remove these people from the bridge.
DOCTOR: You do not recognise me now, but we are known to each other.
ICHTAR: You are mistaken. Take him away.
DOCTOR: No, wait, wait, please. In an earlier regeneration you knew me as the Doctor.
ICHTAR: The Doctor? You can prove what you say?
DOCTOR: I know that you are Ichtar, the surviving leader of the noble Silurian Triad. When we last met I tried to mediate between you and the people of Earth.
ICHTAR: So it is you.
DOCTOR: May we speak?
ICHTAR: The Doctor and Commander Vorshak will remain. Remove the others.
(Sea Devils escort Tegan and Preston out. Two remain to stand guard.)
ICHTAR: I will listen to what you say, but I should tell you that the Silurians have long since abandoned the way of mediation.
DOCTOR: So it seems. Why? A civilised race like yours, waging unprovoked war?
ICHTAR: Defensive war, Doctor. There is a distinction. Silurian law forbids any other.
DOCTOR: Defensive? There's no such thing. When we last met, your supreme wish was to live at peace with the other inhabitants of this planet. Now, why change such an enlightened policy now?
ICHTAR: Our policy has always been peaceful survival. All that has changed is the means by which it can be achieved.
DOCTOR: What, by actions such as we've seen here?
ICHTAR: You forget. Twice we offered the hand of friendship to these ape-descended primitives, and twice we were treacherously attacked, our people slaughtered. It will not happen again.
DOCTOR: There can be no alternative to peaceful co-existence.
ICHTAR: There is, Doctor. A final solution.
DOCTOR: Genocide? When everything you Silurians hold sacred forbids it?
ICHTAR: We will harm no one. These ape primitives will destroy themselves. We, Doctor, will merely provide the pretext.
DOCTOR: You'll trigger the war this Base was designed to fight.
ICHTAR: Yes. And these human beings will die as they have lived, in a sea of their own blood.

[Crew barracks]

(Turlough is trying to open the grill when they hear voices outside.)
TEGAN [OC]: Let go! Let go of me, you (???)
(The door opens.)
TEGAN: Leave me alone! Let go of me.
(Tegan and Preston are pushed into the room and the door is closed again.)
TEGAN: You're alive.
TURLOUGH: Alive, well, and trying to escape. Would one of you mind keeping watch, please?
PRESTON: I will.
(Turlough returns to the grill.)

[Bridge]

TARPOK: Binary circuits of the computer are now functioning.
ICHTAR: Can we activate the missile data banks?
TARPOK: We will need the Commander's hand scan for clearance.
VORSHAK: You'll get no help from me, Silurian.
ICHTAR: Your hand scan, Commander. I will not ask again.
DOCTOR: I would do as he says if I were you, Commander.
VORSHAK: No.
DOCTOR: Alive or dead, they can still use your hand scan, and while there's life there's also hope.
(Vorshak puts his hand on the scanner.)
ICHTAR: Thank you, Doctor.
VORSHAK: Those missiles will never leave their pads, not without a sync operator to complete the firing sequence.
DOCTOR: Not so.
VORSHAK: A sync operator is our insurance against unauthorised launch.
DOCTOR: If my guess is right, that little piece of Silurian gadgetry will more than make up for the lack of a sync operator unless we're able to do something about it.

[Crew barracks]

(Turlough gets the ventilation grill open.)
TURLOUGH: Back to the Tardis.
TEGAN: No, not the Tardis, the bridge. They've got the Doctor up there as well as the Commander. Do you think we can get them both away?
TURLOUGH: What is it about Earth people that makes them think a futile gesture is a noble one?
TEGAN: Turlough

BULIC: You would prefer we left our friends to die?
TURLOUGH: If there was any chance of saving them, I'd be the first to go, but there isn't!
TEGAN: Look, we don't know until we've checked. Coming?
(Bulic gives Tegan a leg up into the ventilation shaft.)

[Bridge]

SCIBUS: The missiles are retargeted.
ICHTAR: Excellent.
DOCTOR: Do you intend to explode those missiles in space?
ICHTAR: Yes.
DOCTOR: Launching those things will trigger a holocaust. You'll destroy everyone.
ICHTAR: The Silurians and Sea Devils will survive.
DOCTOR: To be masters of a dead planet.
ICHTAR: Not quite, Doctor. Still safely hidden away in deep hibernation is the true life force of this planet. When restored, our civilisation will rule the Earth once more.
DOCTOR: I see.

[Crew barracks]

PRESTON: All clear.
(Bulic has already followed Tegan. Turlough tries to give Preston a leg up.)
PRESTON: Turlough.
TURLOUGH: What's the matter?
(He lets her down.)
PRESTON: They would have stood a better chance if we'd gone with them.
TURLOUGH: A better chance of what, of dying? I don't think so. I think they'll manage that very nicely themselves, thank you.
PRESTON: Look
TURLOUGH: Put your foot in.
PRESTON: Really.
(Turlough boost Preston up.)
TURLOUGH: Come on.
(She finally gets into the small shaft.)

[Bridge]

DOCTOR: What about us? Are we included in your final solution?
ICHTAR: We bear you no malice, Doctor. Once we have finished here, you and your companions will be released.
DOCTOR: And the rest of these people?
ICHTAR: They will die. An act of mercy, since there will be nobody for them to rejoin. Commander, you will please join me at the master console.
DOCTOR: Commander.
VORSHAK: Yes, I know.

[Computer room]

(Tegan and Bulic climb down into the room and cross to the door. Bulic looks into the bridge.)
BULIC: The Doctor.

[Bridge]

ICHTAR: Initiate the test firing sequence.
(Vorshak places his palm on the scanner. Bulic slides open the door and the Doctor notices him. He checks that no one else has, and nods. Bulic closes the door.)

[Computer room]

TEGAN: Did he see you?
BULIC: Yes, and he'd better hurry. It can't be long before they discover we've escaped.
(The Doctor moves over to the sync operator's chair but a Sea Devil turns around and he moves back. The Sea Devil looks away and the Doctor backs into the computer room.)
DOCTOR: Excellent timing. Is Turlough safe?
TEGAN: Yes.
DOCTOR: Lead on.
BULIC: But aren't we going to get the Commander out?
DOCTOR: Impossible. They need him in there.
BULIC: Well, we must at least try.
DOCTOR: Not here, not now. We'll come back for him, I promise.
TEGAN: Where are we going?
DOCTOR: The chemical store.
(The Doctor gives Tegan a boost up back into the ventilation shaft.)

[Sea Base]

(Preston and Turlough have escaped.)
PRESTON: This way.
(They collect weapons from a pair of dead guards.)

[Bridge]

SAUVIX: The primitives have escaped.
ICHTAR: Explain.
SAUVIX: Where is the Doctor?
ICHTAR: Commander? Find the Doctor. Find the primitives. Kill them!

[Sea Base]

(Preston and Turlough can hear weapons fire.)
TURLOUGH: They must be mopping up survivors.
PRESTON: Or your friends. Either way, shouldn't we help them?

[Bridge]

ICHTAR: Connect the manipulator.

[Sea Base]

TEGAN: Hurry up, Doctor.
DOCTOR: Shush.
(The two groups meet round a corner.)
TURLOUGH: Doctor.
DOCTOR: Turlough.
TURLOUGH: You didn't think I'd leave you?
TEGAN: It never crossed my mind.
PRESTON: Where were you going?
DOCTOR: The chemical store.
BULIC: This way.

[Bridge]

ICHTAR: Is the computer fully operational?
TARPOK: Restored and tested.
ICHTAR: Then align the manipulator to the computer.

[Chemical store]

DOCTOR: Watch the doors.
(The Doctor goes straight to the racks of hexachromite tanks.)
TEGAN: What are we looking for?
DOCTOR: An alternative to this. Something less lethal that will do the job just as well.
PRESTON: Do you know what the Silurians want?
DOCTOR: Oh yes, all in all they were very forthcoming, really. They intend to launch your missiles and trigger a war to end all wars, between your people, of course.
TURLOUGH: Doctor!
(Everyone scatters and hides as a Sea Devil enters. The Doctor knocks something over and it turns towards him, then fires. The energy bolt punctures a hexachromite tank and the gas kills it, then dissolves its inner organs, which leak out as green goo from its eyes and nose. Everyone comes out of hiding.)
PRESTON: What happened?
DOCTOR: Hexachromite. It does that to all reptile life.
PRESTON: Then use it on the invaders.
DOCTOR: And kill them?
PRESTON: Why not? They're about to start a war that will destroy everyone on Earth.
DOCTOR: I sometimes wonder why I like the people of this miserable planet so much. The Silurians and Sea Devils are noble races. They have skills and talents you pathetic humans can only dream about.
TURLOUGH: That doesn't alter what they're about to do.
DOCTOR: No. No, and we must stop them.
TEGAN: What's the alternative?
DOCTOR: Something that will disable rather than kill.

[Bridge]

TARPOK: Manipulator aligned with the computer.
SCIBUS: Alignment confirmed.
ICHTAR: Good. Then we can proceed.
(The screen flashes - Missile Run Green Alert.)
ICHTAR: Is this a practice missile run?
VORSHAK: It's impossible to tell.
ICHTAR: Check the manipulator.
SCIBUS: Readings confirm a computer controlled practice run.
ICHTAR: It is time to stop this game. Activate the manipulator. Launch the missiles.

[Chemical store]

BULIC: Missile alert.
PRESTON: Doctor
DOCTOR: Yes, I hear. What does it mean?
BULIC: Final countdown is imminent.
TURLOUGH: What are you going to do?
TEGAN: You must decide, Doctor. Billions of people could die.
DOCTOR: Yes, all right. Turlough, get the grill open. Preston, fetch that pump. We have to feed the gas into the ventilation system.

[Bridge]

SCIBUS: Missiles are armed and targeted.
VORSHAK: You're mad.
ICHTAR: The ape primitives have developed this weaponry. We cannot be held responsible for it.
VORSHAK: Contact the heads of governments. Tell them your demands. They'll listen.
ICHTAR: Your people have already had their chance.
VORSHAK: Try one more time, for pity's sake.
ICHTAR: It is too late for pity. It is much too late.

[Chemical store]

DOCTOR: It'll take some time for the gas to spread throughout the Base.
TEGAN: Will it be fast enough?
DOCTOR: I hope it won't be necessary at all. If I can get to the bridge in time, I may be able to reason with Ichtar, persuade him to abandon the missile run.
TURLOUGH: You've tried that already.
DOCTOR: Yes, well, this time I'll have something to back my argument.

[Bridge]

ICHTAR: Soon it'll all be over.
TARPOK: There is computer resistance to the manipulator.
VORSHAK: I said you wouldn't succeed.
ICHTAR: Increase power, Tarpok. It seems that your computers are as stubborn as you are, Commander.

[Chemical store]

DOCTOR: Now, I want you all to go back to the Tardis. You'll be safer there whatever happens.
BULIC: We've gone to yellow alert.
DOCTOR: I must go.
SAUVIX: So, Doctor, I have found you.

[Bridge]

(The intercom beeps.)
ICHTAR: Speak.
SAUVIX [OC]: I have found the Doctor.
ICHTAR: And you have your orders. Kill him.

[Chemical store]

DOCTOR: You must listen to me.
SAUVIX: No, Doctor, you must die. But first, switch off the pump.
(As Sauvix turns his back on her, Preston goes for a weapon. Sauvix turns and kills her, then Bulic lets off a canister of gas at him.)
TEGAN: She's dead.
DOCTOR: A waste.
TEGAN: Well, don't let her death count for nothing.
DOCTOR: No. No, I must get to the bridge.
TEGAN: Doctor, you'll need some help.
DOCTOR: Yes, all right. Bring oxygen. We may need it.
(Tegan and Turlough grab oxygen masks.)

[Bridge]

SCIBUS: The computer has stabilised.
TARPOK: The manipulator has regained control.
(The lights go red.)
ICHTAR: Now do you believe me, Commander?

[Sea Base]

(The group sees a pair of Sea Devils collapse by a ventilation grill.)
DOCTOR: It's working, and quickly. I can't bargain with Ichtar if his guards are dead.

[Bridge]

(The targeting programme comes up on the screen.)
VORSHAK: No!
ICHTAR: Fetch him.
VORSHAK: I will not be responsible for the destruction of my own kind.
(A Sea Devil forces Vorshak's palm onto the scanner.)
ICHTAR: The final phase.
(Bulic connects up another hexachromite canister as it starts to pour into the back of the bridge. The screen says Red Alert. The Doctor, Turlough and Tegan run in.)
ICHTAR: Disarm them! You're just in time to witness the missile launch, Doctor.
DOCTOR: No, wait. Your warriors are dying everywhere. Abandon the base.
TEGAN: Look, it's hexachromite gas.
DOCTOR: Abandon the launch! There's still time to save your own lives.
ICHTAR: There are millions more ready to replace us.
DOCTOR: Who will replace you? With you dies the last of the Triad, the custodians of your race. What will become of your people then?
ICHTAR: You talk in vain, Doctor. Kill them! Kill them!
(But the Sea Devils fall to their knees, dying. The Doctor runs to the intercom.)
DOCTOR: Bulic, turn off the gas!
ICHTAR: Commence ignition, Scibus!
(The Silurians collapse.)
DOCTOR: The missiles are set to fire. How long have we got?
VORSHAK: Less than three minutes.
DOCTOR: What's the abort procedure?
VORSHAK: Well, a phased electrical charge aimed directly at the ignition circuit. It restores the launch to simulation.
DOCTOR: Well, it must be done at once.
VORSHAK: It's impossible. Only a sync operator can do it, and Maddox is dead.
DOCTOR: Well, I shall have to try.
VORSHAK: But the computer would burn out your mind in seconds.
DOCTOR: Oh, you have a better idea?
VORSHAK: Very well, get into the seat.
TURLOUGH: Doctor, try to disconnect the computer first.
DOCTOR: No, no, there isn't time.
VORSHAK: He's right. We're already on final countdown to ignition.
DOCTOR: Tegan, look after the Silurians. Give them oxygen.
VORSHAK: Now, I can perform the manual tasks, but you will have to do the rest.
(The clear helmet sits over the Doctor's head. 94 and counting.)
TEGAN: Good luck, Doctor.
TURLOUGH: The strain's too great. He'll never manage it.
VORSHAK: No. No, his mind is synchronised with the computer. Doctor, can you hear me?
(The Doctor nods.)
VORSHAK: I'm going to switch you through to the ignition circuit.
TURLOUGH: The oxygen isn't having much effect.
TEGAN: This one's coming round.
TURLOUGH: See to the other Silurian there.
(Tegan goes to Scibus at the console.)
VORSHAK: I've isolated the ignition circuit, Doctor. Can you identify it? Nod if you can.
(A nod.)
VORSHAK: Concentrate, Doctor. I will now feed in the charge. To burn out the circuit, concentrate and direct it.
(Tegan's recovering Silurian gets a Sea Devil weapon.)
TEGAN: Turlough!
VORSHAK: The charge must be in phase with the pulse of the circuit. If not, it will destroy you.
TURLOUGH: Look out, Commander.
(The Silurian shoots Vorshak in the back.)
VORSHAK: Now concentrate, Doctor. Let nothing distract you.
(Turlough gets the weapon off the Silurian and shoots it.)
VORSHAK: Now, Doctor!
(Electricity plays over the helmet. The countdown is at 009 when the helmet rises from the Doctor's head.)
VORSHAK: You've done it.
(002, 001, Cancelled.)
VORSHAK: He did it!
(Vorshak collapses and dies across Scibus.)
TEGAN: He's alive.
TURLOUGH: The Commander wasn't so lucky, I'm afraid. He's been shot.
DOCTOR: Did I succeed?
TEGAN: Yes, Doctor.
TURLOUGH: They're all dead, you know.
DOCTOR: There should have been another way.