Lt. (jg) Ken Braden (James Garner) is a US Navy frogman and underwater demolitions expert who is assigned to a vital mission, and to a submarine captained by Commander Stevenson Edmond O'Brien. But Stevenson is a CO who may have seen too many men die -- the two immediately come into conflict over Braden's presence on the boat and his mission, a top secret foray into Japanese waters that jeopardizes the boat. The captain, in his strict adherence to regulations, makes it as difficult as possible for Braden to carry out his assignment, and Braden doesn't make matters easier between them by speaking his mind. And the crew's low morale only makes matters worse as the voyage progresses and the dangers around them mount.