It's another episode from an original series writer, Margaret Armen.
The Enterprise is orbiting an all water planet, caused by volcanic eruptions--they are there to study how and why it happened. Kirk, Spock, McCoy, and Clayton (red shirt alert) go down in an "aquashuttle", which is then attacked by a huge sea creature. They stun it, then run for it. The phasers as usual don't work. They try to beam out but Kirk and Spock are lost. After five days, they are found underwater but alive. How? They have formed gills and webbed hands (of course).
They are brought back to the ship, where a hold is apparently flooded so they have a place to live. McCoy is stumped. Spock realizes there must be intelligent life under the sea. The aquashuttle is destroyed, so Kirk and Spock must go alone. They find the local inhabitants, who tell them to leave. They follow them to an underwater city and sneak in, but are then captured. The tribunal of the "Aquons" determine they are enemies. The Aquon teenagers convince the elders to give them a chance. Meanwhile, a seaquake is coming, so the shuttleboat tries to find Kirk and Spock, which spooks the Aquons. So Kirk and Spock are taken to the surface and left to die.
An Aquon girl saves them by finding the shuttleboat. We learn the Aquon "mutation" is just surgery, and could be reversed. They convince the Aquon girl to help them find the ancient records with the answer. Of course, the sea creature attacks again, but is buried in the ruins. McCoy figures out the antitoxin--it's venom from the sea creature. The Aquon kids help them capture a sea creature (why not just use the buried one?), and McCoy figures out the antitoxin. The Enterprise diverts the seaquake with phasers, and all is well.
Some things to look for:
- The Enterprise now carries an aquashuttle that can fly in space and under the sea--maybe they borrowed it from the Thunderbirds
- They also have a boat for surface use
- Always alert to save money, Filmation shows the shuttle being attacked by spinning and shaking the animation cel
- We also hear a bowler getting a strike when the ship hits the ocean floor
- Kirk: "I can't command a ship from inside an aquarium!"
- I suspect they borrowed some footage from the Aquaman Filmation series--the swimming effects look the same
- We hear a lot of the Filmation voice actors playing the Aquons
- I love Scotty floating in the water with his lifebelt glow
- Wavy underwater effects come and go--sloppy animators!
- The ancient records are found in a sealed room w/o water--but Kirk and Spock don't seem to be affected
- Lots of weird effects as Kirk takes the antitoxin--his skin changes color, he grows scales and a fin, then back to normal
Remember, you can play along by watching the show on Netflix!