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.
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