Kirk and Spock are in a submersible shuttle craft investigating a planet that is almost all water. It is subject to severe "earthquakes" that have in the past caused massive amounts of landmass to be lowered into the sea.
While skimming the surface and gathering data, the shuttle craft is attacked by a giant sur-snake. They are able to repel the first attack, but when it attacks again, it destroys the shuttle craft. Kirk and Spock are thrown from the craft and when they recover, they are now aquatic, unable to survive on land. To have any hope of being human again, they must go back under water and seek out the intelligent humanoids that live there.
After a debate among the leaders of the aquatic creatures, they agree to let Kirk and Spock go to their submerged hall of records to search for information regarding how to transform back. The aquatic creatures are weary of air breathers, for there was apparently hostile feelings between the two groups in the past.
The problem with this episode is the ease with which the transformation takes place. There are no side effects or consequential traumas to what would have to be a massive alteration of body structure. The acquisition of the antidote is also a bit overplayed.