The U.S.S. Enterprise arrives to study the planet Argo—a water-based planet where earthquakes caused the land masses to sink—for the benefit of other Federation worlds that will encounter similar quakes. The crew lands in a specially designed aqua-shuttle but is immediately attacked by a sea monster and Kirk and Spock become trapped in the monster's grip.

Five days later, McCoy, Scott, and Lt. Clayton find the missing officers; they have been transformed into water breathers by the monster. Kirk and Spock explore the submerged Argo and find a group of inhabitants who hate any air-breathers. Hesitantly, the Aquans agree to perform surgery to return Kirk and Spock to normal, but when Scotty dives down to warn of an impending quake, they become fearful of air-breathing spies. Kirk and Spock are deemed treacherous and are sentenced to death. Rila, a water breather, agrees to help them if they will in turn help him find a buried city and a way to reverse the mutation.

They are successful and obtain the necessary medicine to help Kirk and Spock, but Kirk nearly dies from the vaccination. In orbit, the U.S.S. Enterprise uses its phasers to hit the uninhabited sections of the planet in an attempt to move the quake's epicenter away from the Aquan city. Grateful for saving their lives, the young Aquans make their records available to the Federation and decide to use the new found medicine to reverse their water-breathing mutations and resume life above ground.

Cast:

William Shatner as James T. Kirk
Leonard Nimoy as Spock
DeForest Kelley as Leonard H. McCoy
James Doohan as Montgomery Scott
Nichelle Nichols as Uhura
George Takei as Hikaru Sulu

Guest Cast:

James Doohan as Lieutenant Clayton
James Doohan as Domar
Majel Barrett as Rila
James Doohan as Cadmar
James Doohan as Lemus
David Gerrold as Nephro
Majel Barrett as Aquan female voice
James Doohan as Aquan male voice
James Doohan as Lieutenant Arex

Creative Staff:

Written By: Margaret Armen
Director: Hal Sutherland