Stardate: 5499.9--The Enterprise arrives to study the planet Argo, a water-based planet where earthquakes caused the land masses to sink.
While exploring an aquatic planet, Kirk and Spock are lost at sea. When they are found they have been transformed into amphibious life forms. Now they must track down the planet's mysterious inhabitants and find a cure, or be left to live as freaks in the Federation.
original airdate--December 01,1973
Goof: After Kirk's mutated hands are shown to heal, they are again mutated in a later scene.