The U.S. nuclear submarine Montana unfortunately disappeared while tracking an unidentified object in the deep sea. Bud (Ed Harris), the foreman of the high-pressure diving rig Deep Sea Heart, was ordered to cooperate with Captain Coffey (Michael Biehn) to rescue it. Unexpectedly, Bud's wife, Lindsey (Mary Elizabeth Mastrantonio), an engineer of the oil company, who had been quarreling with him, also went with him. The three-member rescue team dived to the edge of the abyss where the submarine disappeared, but found that no one survived on the submarine. What was terrifying was that there was an unknown monster in the abyss. After a series of crises, Captain Coffey did not listen to the dissuasion and believed that this was a Soviet conspiracy. He decided to launch the nuclear bomb removed from the submarine to an unknown enemy, so Bud and Lindsey joined forces to start a desperate fight. At the critical moment, Bud saved Lindsey regardless of danger, and dived into the abyss to remove the fuse on the nuclear warhead. After completing the task, Bud's oxygen was exhausted, and at this moment the monster in the deep sea appeared again.