In the midst of the tense Cold War, a U.S. Navy nuclear submarine is on a crucial mission in the Pacific Ocean. The crew, made up of brave sailors, is under the command of the strict Captain Frank Ramsey, a veteran who has seen more than he should and does not hesitate to make tough decisions. However, the calm of the mission is interrupted when they receive a cryptic message that could be an indication of an imminent attack. As tension rises, the first officer, the young and bold junior lieutenant Ron Hunter, begins to question the interpretation of the message.
As the internal conflict grows, the loyalty and discipline of the crew are put to the test. Ramsey, confident that they must launch retaliatory missiles, faces off against Hunter, who argues that the message may have been sent in a context that does not justify an immediate military response. The two men become rivals, unleashing a verbal and philosophical battle over duty, honor, and the cost of war. In this atmosphere of claustrophobia and adrenaline, the crew must navigate not only the dangers of the deep ocean but also the threat that a captain and his first officer can pose to each other. The tension culminates in a final confrontation, where deadly decisions must be made in a matter of seconds, and the fate of many hangs by a thread.