Milquetoast Henry Limpet experiences his fondest wish and is transformed into a fish. As a talking fish he assists the US Navy in hunting German submarines during World War II.

Don Knotts
as Henry Limpet

Carole Cook
as Bessie Limpet

Jack Weston
as George Stickel

Andrew Duggan
as Harlock

Larry Keating
as Admiral P.P. Spewter

Oscar Beregi Jr.
as Nazi Admiral (as Oscar Beregi)

Charles Meredith
as Fleet Admiral

Elizabeth MacRae
as Ladyfish (voice)

Paul Frees
as Crusty (voice)

Wesley Gale
as Man Staring Into Window (uncredited)