A World War II 4-F saves the U.S. Navy when he's transformed into a dolphin.

Henry Limpet, a mildmannered Brooklyn bookkeeper, irritates his wife, Bessie, because his love for fish is so strong that he yearns to become one. His attempt to join the Navy fails, and matters are made worse when his best friend, George Stickle, is accepted. One day Henry, Bessie, and George go to Coney Island; Henry falls off a dock, disappears into the ocean, and miraculously is transformed into a dolphin. After making friends with a sea snail, he falls in love with Ladyfish, a female dolphin. With the outbreak of World War II, he guides the U. S. sub chasers in tracking down and sinking German U-boats, and although baffled by their success, the Navy bestows a commission upon dolphin Limpet. At the war's conclusion, he surfaces for a poignant farewell to his wife and swims off with Ladyfish.