It is 1961 and Captain Alexi Vostrikov (Harrison Ford) has just taken over command of the USSR's first nuclear submarine, K-19, from Captain Mikhail Polenin (Liam Neeson), who has been demoted to executive officer after stating that the vessel was not yet ready. His fears are justified when various equipment either goes missing or never materialises, but Vostrikov is ordered by his superiors to sail into the North Atlantic for a missile test - which would alert the USA of the nuclear fire-power now in the hands of their enemy. When a leak on board threatens the nuclear payload, Vostrikov realises that if it blows it could be seen as the beginning of the Third World War and orders the crew to take it in 10 minute shifts to repair the damage - thus exposing all the men to radiation.