A US Navy submarine, the USS Corsair, is operating in the North Atlantic, hunting German merchant raiders that are preying on Allied shipping. Its new executive officer, Lt. Ward Stewart, has been transferred back into submarines after commanding his own PT boat. At the submarine base in New London, Connecticut, he asks his new captain, Lt. Cmdr. Dewey Connors, for a weekend leave to settle his affairs before taking up his new assignment. On a train bound for Washington D.C., Stewart accidentally encounters New London school teacher Jean Hewlett and her students. Despite her initial resistance to his efforts, he charms her and they fall in love.

The crew of the nuclear submarine USS Ulysses rescues supposed victims of a boat disaster, but the victims turn out to be terrorists intent on capturing nuclear weapons aboard the sub. Only a former SEAL, now a submarine consultant, can save the crew by sliding aboard while the sub is underwater. The term "crash dive" refers to the sudden dropping of a sub to escape detection, an act that a nuclear sub is never supposed to make. Of course, it comes into play in this film.

US Navy medical officer Jake Fuller is assigned to a goodwill visit aboard a Russian submarine. But he and his companion, Lt. Swain, end up alone among terrorists, who have taken over the submarine and threaten to fire its nuclear weapons.