This is another of the Animated Series episodes where they went to town by doing a story that they never could have done with the live action series due to impossibility or expense of the special effects. If only they'd come up with a better story.

This was written by Margaret Armen, who also wrote The Lorelei Signal, and the original series episodes The Gamesters of Triskelion, The Paradise Syndrome, and The Cloud Minders. That's a pretty impressively bad hit rate.

Transcript

1
Captain's log, Stardate 5499.9. Studying planet Argo, where seismic activity submerged almost all the land.
Spock: Geologically that should not be—
Kirk: It's just like Atlantis!
Spock: That is mythol—
Kirk: Atlantis!
2
{on Argo}
Spock: Fascinating. A giant fish-creature has our aqua-shuttle.
Kirk: Summon bigger fish!
3
{Enterprise sick bay}
McCoy: You've been mutated into water-breathers!
Spock: Biology does not—
Kirk: We have two options. Live in a tank, or on Argo. I don't accept either!
Spock: Logically, you must accept—
Kirk: Neither!
4
{Argo, on the ocean floor}
Domar: You are air-breathing spies. We made you into water breathers so you can't spy on us!
Spock: That makes no—
Domar: Spies!
5
Rila: We groovy young merpeople are free thinkers, unlike our elders. We'll help you get the anti-toxin you need to demutate.
Spock: An anti-toxin cannot—
Kirk: Awesome!
6
{Enterprise bridge}
Kirk: Now we're back to normal, we'll save you from an earthquake by moving its epicentre with our phasers.
Spock: Why do I even bother?