In 1942, in California, the diver Lieutenant Kenneth M. Braden from the navy demolition team falls in love with Sally Johnson and proposes to her. The reluctant woman asks for more time to decide, but Braden is summoned to travel to a base in the Pacific.

Soon he discovers that Sally actually works for the Navy and has been assigned to check his qualifications for a a classified mission. He must travel on board the submarine Barracuda commanded by Commander Paul Stevenson to a Japanese island and photograph their secret radio codes in a very dangerous mission.