U. S. Navy Comdr. James Ferraday, stationed in Scotland, receives orders from Admiral Garvey to take his nuclear submarine to a British North Pole weather station called Ice Station Zebra. Ferraday's mission, which he does not yet know, is to recover a capsule from a grounded Russian space satellite containing reconnaissance photographs of all U. S. and Russian missile sites. Also aboard the sub are two British agents, David Jones and Boris Vaslov, the latter a communist defector, and two U. S. Marine officers, Lieut. Russell Walker and Capt. Leslie Anders. En route, the vessel is sabotaged and almost exceeds its implosion depth before the crew can repair the damage and regain normal depth. Ferraday's suspicions that Vaslov is responsible are rejected by Jones, who vouches for his associate's loyalty and, instead, accuses Anders of sabotage. Once the sub reaches Ice Station Zebra, a search party finds only fire-gutted buildings and the frozen corpses of the base personnel. As the search for the capsule begins, Ferraday learns from the sub's radar that Russian aircraft are approaching. Jones is knocked unconscious and recovers to find Anders and Vaslov fighting; assuming that Anders is the spy, Jones kills him. Later, as the capsule is recovered, Russian paratroops under the command of Colonel Ostrovsky land in the area. Vaslov now reveals his traitorous nature by attempting to hand the capsule over to the Russians, but Jones hurls Vaslov against an iceblock and strangles him. Though Ferraday is obliged to give up the capsule, he destroys it as the Russians are lifting it by recovery aircraft. With the photographs lost to both sides, Ferraday and Ostrovsky agree that the incident shall be publicized as an example of the friendly cooperation between two great nations.