Aquatic life fanatic Henry Limpet (Don Knotts) falls into the sea and transforms into a fish, quickly becoming pals with a crab and developing a romantic bond with a fetching female fish. As a human, Henry was passed over for naval service. But now that he's a fish, he seeks out his friend Lt. George Stickel (Jack Weston). Finding him on a small boat in the Atlantic, Henry explains that he can alert sailors when enemy ships are close by, and soon he's helping America win World War II.

Bill, Jeff and Judy begin building rafts to help the human tribes reach New Valley; while going through a nearby jungle, the trio encounters a very unlikely sight: a wrecked U.S. spaceship and its pilot, Ron Brent.