During the Cuban Missle Crisis, a Russian sub is sunk while en route to Havana. As the sub goes down, the hold is breached and barrels full of some mysterious substance tumble out. Years later, an American nuclear submarine is transporting a captured terrorist to the States. The terrorist's henchmen, however, are planning to hijack the sub and rescue their leader. Meanwhile, a large, unidentified creature is approaching the sub at high speed...