In the midst of the Cold War, the Soviet Union launches its first ballistic missile nuclear submarine, christened K-19. Commanded by the stern Captain Alexei Vostrikov, who replaces the beloved original captain Mikhail Polenin just before deployment, the vessel is pushed to its limits on a test mission. Vostrikov is determined to prove the submarine's operational capability to his superiors, insisting on rigorous drills and preparation. Despite the crew's loyalty to Polenin, they are forced to comply with Vostrikov's demanding standards, setting the stage for a harrowing voyage. The submarine's mission is critical to demonstrate Soviet naval strength, but it is plagued by construction issues and equipment failures, reflecting the rushed engineering efforts during its construction.

As the K-19 approaches its testing zone in the North Atlantic to fire a test missile, it experiences a catastrophic nuclear reactor malfunction, threatening not just the lives of the crew but also the possibility of a nuclear disaster that could escalate tensions between the Soviet Union and the United States. Faced with limited options and a malfunctioning reactor, Vostrikov and Polenin must set aside their differences to prevent a catastrophe. The crew is pushed to their physical and psychological limits as they endeavor to fix the reactor with makeshift solutions, battling radiation exposure and the freezing cold. Their struggle represents not just a fight for survival but a profound test of human spirit and loyalty.