On a routine peacetime mission, the crew of HM Submarine Trojan runs afoul of an electrically operated mine, leftover from the war. An unavoidable explosion sends the ship plunging to the seabed. Emergency signals are sent, and the Captain assembles the survivors to make an escape plan. Some can escape, but others will have to wait for the rescuers to arrive. Those left must struggle with fear and illness whilst the rescue team attempts to reach the vessel. To make matters worse, there is a storm brewing.