While exploring the water planet Argo, Captain Kirk and Mr. Spock are transformed into water breathers by the planet's undersea inhabitants, the Aquans. In order to return to their normal selves, they must enlist the help of the Aquans to capture a giant sur-snake, whose venom holds the antidote.

William Shatner (Captain James T. Kirk)
Leonard Nimoy (Commander Spock)
DeForest Kelley (Dr. Leonard H. McCoy)

George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Majel Barrett (Junior Tribune Rila)
James Doohan (Lt. Cmdr. Montgomery Scott)
Lou Scheimer (Junior Tribune Lemus)

• Nichelle Nichols is credited as a co-star as always, but does not have any speaking roles.