On the water planet Argo, Kirk and Spock are turned into water-breathers and must win over a fearful undersea people to be changed back.
Seismic upheaval has turned the planet Argo into a world of oceans. While Kirk and Spock explore in an aquashuttle, a giant sea creature attacks them, and when they are found they have been changed into beings who can only breathe water. McCoy concludes that intelligent life must still exist beneath the sea.
The two officers find the Aquans, an advanced undersea people who fear air-breathers, and are accused of spying. A sympathetic council member, Rila, helps them and reveals that the change can be reversed. Kirk persuades her to help find the lost formula and capture a giant sur-snake whose venom is needed for the antidote. Margaret Armen wrote the episode.