[Shuttlecraft]

(A special shuttlecraft modeled on a catamaran has been launched from Enterprise and landed on the ocean)
KIRK: This is the largest land mass on the planet, isn't it, Spock?
SPOCK: Yes, Captain. The others are mere outcroppings. They suggest the peaks of submerged mountains.
MCCOY: I suggest that we get these seawater and algae specimens. They're the least of the lot.
SPOCK: For once I agree with you, Doctor. The more informative specimens will come from the sea bottom.
KIRK: Lieutenant Clayton, open the hatch and let our two scientists here get on with their business.
CLAYTON: Aye, sir.
MCCOY: What's that?
(Something with huge tentacles picks up the shuttlecraft and throws it away)
KIRK: Phasers on stun, Mister Spock.
SPOCK: Firing, Captain.
(The creature retreats below the surface)
MCCOY: What the devil was that thing?
SPOCK: Apparently a life form which previous surveys have not recorded.
KIRK: Let's get a closer look at it before the phaser stun wears off. Submerge, Lieutenant.
CLAYTON: Aye, sir.
(They descend through the green water to the sea bed)
MCCOY: Dual respiratory system. Lungs and gills.
SPOCK: Its skeleton is similar to that of a Denebian whale, Captain.
(The shuttlecraft gets slapped across the sea bed)
KIRK: It's regaining consciousness. Surface, Mister Clayton.
CLAYTON: Aye, sir.
(The shuttlecraft is chased along the surface)
KIRK: Spock, prepare to fire phasers.
SPOCK: Phasers do not respond, Captain. We must have sustained damage to the power pack when the creature struck us.
KIRK: Kirk to Enterprise. Emergency.
SCOTT [OC]: Enterprise. What is it, sir.
KIRK: We're under attack. Beam us aboard, Scotty.
(But the sea monster bites the shuttlecraft then throws it against rocks. McCoy falls into the water, followed by Clayton. Spock and Kirk are unconscious in the craft)
SCOTT [OC]: We've lost contact, Captain.

[Bridge]

SCOTT: Repeat, we've lost contact. Come in, Captain. Come in.
(The monster grabs the shuttlecraft and takes it underwater)

Ship's log, stardate 5504.2. Lieutenant Commander Scott in command. Captain Kirk and First Officer Spock have been missing for almost five days. Search parties can find no trace of them or the aqua-shuttle. Doctor McCoy and I are continuing the search at a greater distance from the attack point.

[Boat]

MCCOY: There's nothing. There's just nothing.
CLAYTON: I see some rocks. They're just above sea level, about two miles away. There's something on them.

[Rocks]

(It's the aqua-shuttle, mangled and smashed. Nearby, Kirk and Spock are lying face down in the water)
MCCOY: Maybe they're still alive.
(They pull them out of the water)
MCCOY: Their bodies are functioning. Metabolism, heart. Everything but the lungs.
KIRK: Can't breathe. Suffocating. Help. Can't breathe.
(He lifts a webbed hand to his throat)
SCOTT: What happened to them, Doctor? What is it?
MCCOY: Something's changed their whole lung structure. They can't live in the air anymore.

Medical log, stardate 5506.2. Captain Kirk and First Office Spock were rescued forty eight hours ago. They have no recollection of what happened to them after they were attacked, but medical examinations show and unidentified substance in the bloodstream has affected their entire metabolism and changed them into water-breathers. Their internal structure is completely transformed, and even their eyes are covered with a transparent film like the second eyelid of a fish. So far all efforts to return them to normal have failed.

[Aquarium]

(Kirk and Spock are in water, Scott and McCoy the other side of the glass)
MCCOY: We're stumped, Jim. The hormone seems to be the major factor in your mutation, but we just can't identify it.
KIRK: Are you sure the mutation wasn't somehow accomplished naturally?
MCCOY: It was infused into your bodies at key structural points. It had to be done by injection.
KIRK: But can't you duplicate it, Bones? Your medical computers have information on every procedure in the Federation's hospitals.
MCCOY: It's highly sophisticated and completely unknown to us.
SPOCK: That implies there are intelligent life forms on Argo.
MCCOY: The sensors don't indicate any life forms down there, aside from fish.
SPOCK: They did fail to indicate the large marine creature that attacked us. Perhaps the readings are incomplete, blocked by underwater pressure forces. Oceans are less turbulent at greater depth. Such conditions would be more suitable to intelligent forms.
MCCOY: Well, we can't do much underwater exploration without the aqua-shuttle. Our equipment is too limited.
KIRK: We aren't limited. We can go wherever we want in that ocean.
MCCOY: It's too risky, Jim. Argo's almost totally unexplored. You'd be out of contact range, unarmed.
KIRK: I have to, Bones. Look at this place. A tank. I can't command a ship from inside an aquarium. I can't live in here.
MCCOY: I know what you're saying, Jim, but if you go down on that planet and you die there
KIRK: Right now we have only two choices. Live in an aquarium somewhere for the rest of our lives or stay on Argo. I won't accept either one of them.
MCCOY: What about you, Spock? What does your logic tell you?
SPOCK: The Captain states the case emotionally, of course, but correctly. I would be of little value to this ship if I am confined to a tank in Sickbay. I must accompany the Captain.
KIRK: An intelligence that can produce this kind of mutation can change us back. We've got to find it, Bones. It must be there.

[Boat]

KIRK: We'll make contact as soon as possible, Mister Scott.
SCOTT: Aye, sir.
(Kirk and Spock remove their aqua-lungs and dive into the ocean)

[Sea bed]

(A group of Aquans are gathering food when they see the uniformed and booted humans swimming towards them. Aquans are green, with gills and dark green fins across their heads, the sides of their necks and chests)
MALE: Go away, air breathers. You're not wanted here.
KIRK: We won't harm you. We're friends.
FEMALE: Leave us. It is enough our young saved your lives once. If you go on, nothing will save you again.
KIRK: Wait. Listen.
MALE: Go away.
(The Aquans leave)
KIRK: I don't understand. They said they'd saved us once.
SPOCK: I believe the phrase was, 'our young saved your lives once.'
KIRK: Yes, and they didn't seem to approve. The answer appears to be in that direction, Spock.
SPOCK: I would say so, Captain. The instinctive reaction of all frightened creatures is to retreat to a place of safety.
(They descend into the rift in the seabed, and follow a tunnel to)
SPOCK: Fascinating. And beautiful.
KIRK: An undersea civilisation capable of fantastic architecture. Medical triumphs unheard of in our galaxy. Why should they be afraid of us, Spock?
SPOCK: It is quite possible, Captain, that they find us grotesque and ugly. And many people fear beings different from themselves.
KIRK: Well, speculation isn't much help. We have to get in there.
SPOCK: Perhaps there is a way open on the far side. There is much less activity there.
(On the other side of the city)
KIRK: That building in the centre. It seems to be important.
(Then they are caught in a net)

[Chamber]

These are the spies, Tribune. Air-breathers.
DOMAR: You stand before the Ruling Tribunal of the Aquans. I am Domar, the High Tribune.
KIRK: I'm Captain Kirk of the starship Enterprise. This is my first officer, Mister Spock.
DOMAR: You are air-breather enemies from the surface. We have expected spies for a long time.
SPOCK: We came here in peace, Tribune.
CADMAR: The ancient records warn that air-breathers never come in peace.
RILA: They came without weapons.
MALE: Can we do nothing without consulting the ancient records?
CADMAR: Why do the Junior Tribunes wish always to change the records?
DOMAR: Let the mesh be removed, but stand ready.
MALE: The surgo-op was excellent. They have mutated perfectly.
KIRK: Your scientists induced these mutations?
DOMAR: We do not wish to kill, even air-breathers. Your memory was eradicated so you could not return here.
CADMAR: It would seem their ancient records are as well preserved as ours. They found us again to come among us as spies.
RILA: You do not give them a chance to defend themselves, Cadmar.
DOMAR: Rila is correct. You may speak.
KIRK: We are visitors to your planet. We come from another world.
RILA: You do not live on the surface places?
SPOCK: Not on this world.
CADMAR: Enough! Clearly, this is a lie. The air-breathers have come again to destroy us.
SPOCK: You are mistaken, sir. Our only purpose here is to find the means of reversing our mutations.
DOMAR: That is impossible. Only Aquan mutation is designated by the surgo-records.

[Boat]

MCCOY [OC]: Enterprise to Mister Scott.
SCOTT: Scott here. What is it, Doctor?
MCCOY: Sensors just gave us an update reading, Scotty. There's a seaquake due in that area, a bad one. Complete topography changes.
SCOTT: How soon?

[Bridge]

MCCOY: Within four hours. When are Jim and Spock due to make contact?
SCOTT [OC]: About the same time.
MCCOY: Can you contact them sooner?
SCOTT [OC]: We can try like blue blazes. Scott out.

[Boat]

SCOTT: Their directional marker indicates they descended toward that fern bed.

[Underwater]

(As the hull of the boat passes overhead)
FEMALE: Air-breathers. We must inform the Tribunes.

[Chamber]

KIRK: The name of our starship is on the aqua-shuttle. If you want proof, examine the wreckage.
RILA: Yes. Let us examine the wreckage before we make judgement.
FEMALE: We have news, High Tribune.
DOMAR: Speak it.
FEMALE: Three air-breathers have invaded the sea foliage. We saw them as we gathered edibles.
DOMAR: Do you still believe these creatures came in peace?
RILA: We do not know what to believe.
DOMAR: Take these spies to the surface boulders and leave them there. It is justice enough for our enemies.

{Rocks]

(Spock and Kirk are pinned under a net, near the aqua-shuttle remains)
RILA: Conserve your strength. I will free you. The mesh is too strong.
KIRK: Go to the big island. Bring assistance party.

[Boat]

(Scott has been diving)
CLAYTON: Did you see anything?
SCOTT: Not a sign of them. Wait a minute. Who are
RILA: Follow me. Your friends need help.

[Rocks]

RILA: There.
SCOTT: Quick, lads.
(Spock and Kirk are freed and back in the water)
SCOTT: Stand by in the scouter-gig, Lieutenant.

[Underwater]

(Scott is protected by a personal force field)
KIRK: Rila, this is my engineering officer, Mister Scott. Tribune Rila is an Aquan. Their city is a short distance away.
SCOTT: We're obliged for your help. Captain, we've been trying to contact you for two hours. There's a sea bottom quake due. It'll destroy this whole area.
RILA: There are legends of such a happening. It destroyed the ancient knowledge and caused the great surface places to sink into the sea.
SPOCK: That suggests your race evolved from its ancient enemies.
RILA: We did not evolve, Mister Spock. When the surface places began to sink, many air-breathers were mutated through surgo-op, just as you were. As the centuries passed, the mutations became hereditary.
KIRK: Strange that such a highly developed race would be violent.
SPOCK: Apparently they were exposed to a frightening geological disaster. Such hardship induces savagery and violence.
RILA: They hunted and killed us. We feared they would contaminate us with their violence. That is also why it is forbidden to mutate back to surface forms.
KIRK: Then reverse mutation is possible.
RILA: There are legends of sealed places in the sunken ruins where many knowledge records of it are kept.
(The seabed shakes)
SCOTT: There's not much time, Captain. The quake's due in less than two hours.
KIRK: How far are the ruins, Rila?
RILA: Only a short distance.
KIRK: Those old records may hold the key to reversing our mutations. We've got to find them before it's too late.
RILA: I cannot take you to the ruins. It is against ordainments.
KIRK: It's important, Rila. Not just for us. The population of another planet is threatened by surface changes, too. Argo's ancient knowledge can help save them.
RILA: Yes. I will take you as far as the reef barrier.

[Barrier]

RILA: The ruins lie that way. Now, take care. The tides are strong.
(She leaves. Kirk and Spock are pulled along by the current)

[Ruins]

SPOCK: Fascinating. Obviously, an entire portion of the continental shelf sank within a few minutes. This may be the sealed place Rila mentioned. I believe this is an entrance panel, Captain.
(They swim down a staircase)
KIRK: There's another chamber.

[Chamber]

(A room in the Roman style, with canisters and equipment including a caduceus)
SPOCK: This resembles ancient medical symbols, Captain. The torso indicates mutation.
KIRK: That's it, Spock. The key to the survival of the race.
(They grab a couple of canisters each and swim for it. But they encounter Mister Tentacles, the sea monster. Fortunately another quake hits, and it gets buried under a collapsed building)

[Aquarium]

(McCoy has got the scrolls out of the canisters)
MCCOY: If the universal translator is right, the substance in your bloodstream is similar to the ambergris of Earth whales. Reverse mutation is induced by infusion of an antitoxin, concentrated doses of it.
KIRK: What's the composition of the antitoxin, Bones?
MCCOY: It's made from the venom of the Argo sur-snake.

[Underwater]

KIRK: To capture a sur-snake, we need your help. Our people can't descend to that depth.
NEPHRO: Domar has warned that if we break more ordainments, we will be exiled to the open seas.
SPOCK: We cannot reverse our mutations without the serum, nor can the Aquans benefit from the knowledge of their ancestors.
NEPHRO: But the ordainments state clearly that capturing sur-snakes is forbidden.
MALE: Ordainments are useless in times of turbulence. Only knowledge will help us.
RILA: Nephro, we must break the ordainments.

[Ruins]

(They net the monster that was caught by the falling ruins)
RILA: Quickly, Captain.
(They milk it's venom while another quake strikes)
RILA: Quickly, we must leave.
KIRK: We need more venom.
RILA: No, no, there is no time.
(This time the monster is totally squashed by falling masonry)

[Aquarium]

(McCoy and Chapel are in the water too)
MCCOY: We have prepared an antitoxin from the venom of the sur-snake, but a part of the old records is missing, so the exact dosage is unknown. Experiments on fish show that if the antitoxin is too strong, it causes over-mutation, which cannot be reversed by any means. We're running out of venom, so conclusive experiments are impossible. I decreased the strength by three quarters, Jim, but we can't be sure it's right.
KIRK: How many infusions, Bones?
MCCOY: Two small, one large.
KIRK: Well, let's get started.
(McCoy injects Kirk, who turns red then white)
MCCOY: His pulse is fading. (turns pink again) Pulse and heart normal. Something's happening to his metabolism. It's at least ten times normal speed. (the webbing between his fingers disappears) Metabolism normal. Heartbeat normal. The lungs are beginning to revert. This is the last dose, the largest infusion. (he grows scales) The antitoxin was too strong. (The scales vanish) He's starting to breathe. Quick, get him out of the tank!
(Outside)
MCCOY: How do you feel, Jim?
KIRK: A little dizzy. Otherwise normal.
MCCOY: Well, better get into some dry clothes. I wouldn't want you to catch cold. Your turn, Spock.

[Bridge]

(Domar and Rila are wearing aqualungs)
KIRK: We'll bombard the unpopulated pressure areas with phaser beams and change the epicentre of the quake so it won't endanger your people or your city.
DOMAR: I did not believe such knowledge was possible.
SCOTT: Major tremors will start in three minutes, Captain.
DOMAR: It is incredible. All of it.
KIRK: Confirm co-ordinates, Mister Spock.
SPOCK: Confirmed, sir.
KIRK: Fire phasers as directed, gentlemen.
AREX: Aye, aye, sir.
SPOCK: Sensors indicate the epicentre of the quake was in the north polar seas, Captain, a totally uninhabited area.
DOMAR: Then our people are safe.
KIRK: When can we beam down, Spock?
SPOCK: The seabed should stabilize within a few hours.

[Shoreline]

SPOCK: Argo appears to have a new city, Captain, or rather, one reborn.
RILA: Our people survived the seaquakes with little damage, thanks to your help.
DOMAR: We owe you and your friends much gratitude, Captain Kirk. We will try to repay by making the ancient records available for your studies.
SPOCK: Our experience here has shown us the means of diverting quakes. That will mean much to the endangered Federation planet.
RILA: The light is warm and the air is soft. I shall be glad when the surface places can be inhabited.
DOMAR: The young Aquans have decided to rebuild the great shelters of the air-breathers.
KIRK: Only the young Aquans?
DOMAR: Senior Aquans cannot adjust to the thought of becoming air-breathers. We will remain at the Aqua City.
KIRK: Don't lose contact with each other like your ancestors.
DOMAR: We will pass ordainments to forbid it.
RILA: And we, this time, will not ignore them.