Captain Cassidy (Cary Grant) is in charge of a submarine on a mission so secret even he doesn't know what it is until his ship departs San Francisco and he opens his sealed orders. His task -- to pick up a meteorologist (John Ridgely) and travel to the coast of Japan to gather weather information for the Doolittle Raid, the first air strike on Tokyo. Along the way they fight to elude detection from Japanese ships and eventually face off against an enemy aircraft carrier.