In 1942, in Norway, Nicole Larsen is detested as a traitor by her countrymen for her allegiance to the Nazi cause and her romantic involvement with Major Paul Dichter, a Nazi officer. In reality, Nicole is an agent of the Norwegian underground and has been invaluable in pinpointing strategic Nazi targets for the Allies. While reporting to her contact, Dr. Aanrud, one day, Nicole is cautioned that Dichter must be killed as soon as he comes to mistrust her. Dichter, pressured by his commanding officer, Col. Kurt Von Elser, to find the leak in his security, begins to question Nicole. When Nicole sends word of Dichter's suspicions to London headquarters, Captain Allan Lowell, who has been assigned to bomb a German ammunition dump in Slavik, a coastal Norwegian town, volunteers to assassinate Dichter as part of his mission. Upon landing in Norway, Allan and his Norwegian confederate, Ole, seek refuge in a farmhouse owned by Ole's cousin. When the man makes an excuse to leave the house later that night, Allan sends Ole to follow him. In the barn, Ole watches increduously as his cousin radios Nazi headquarters about the arrival of the two commandos. Ole attacks and kills his cousin but is too late, because a truckload of Nazi soldiers has been dispatched to apprehend the spies. The Nazis pursue the commandos into the woods, where they kill Ole and wound Allan. Delighted by the news of Allan's capture, Dichter eagerly anticipates forcing his prisoner to expose the traitor in his organization. Dr. Aanrud then contacts Nicole to warn her that the British prisoner must be removed from his hospital bed before the Nazis force him to talk. To accomplish this, he instructs Nicole to enlist the aid of Rose Linstrom, a nurse in the prison ward. To create an excuse to visit the hospital, Nicole's faithful servant and fellow collaborator Soren cuts his wrist. Nicole accompanies him to the hospital, and when she meets Paul in the lobby, he takes her with him to the prison ward. There, Nicole contacts Rose and recognizes the British prisoner to be the man she fell in love with years earlier while on holiday. That night, Rose smuggles Allan out of the hospital and hides him in the basement of Nicole's house. Later, Rose is detained and questioned by the Nazis, and although they threaten her family, she refuses to cooperate. Reunited with Nicole, Allan admits that he knew that she was the agent in danger and has come to rescue her from Dichter. Nicole then recounts how the horrors inflicted by the Nazis transformed her from an innocent young girl into a patriot willing to die for her country. Meanwhile, at headquarters, Von Elser holds Dichter responsible for the spy's escape and berates him for failing to search Nicole's basement. To defend himself, Dichter announces that he has complete faith in Nicole and has asked him to marry her. Later that night, Dichter proposes to Nicole and insists that the wedding be held on Thursday, the day of the raid. When Nicole cites Dichter's proposal as proof of his trust in her, Allan realizes that the major has fed her a piece of misinformation to test her loyalty about a cannery providing cover for a German munitions factory. Nicole has already transmitted the information, however, and refuses to retract it, believing that it may be true. On Thursday night, Allan prepares to join the commando raid as Nicole, dressed in her bridal gown, bids him a tearful farewell. Surrounded by his Nazi compatriots at the wedding hall, Dichter receives word about an Allied bombing raid on the cannery and realizes that Nicole has betrayed him. After the ceremony, Dichter insists on returning to Nicole's house and there denounces her as a spy and informs her that she will die in an auto accident that night. As she spits out her contempt for Dichter, Allan shoots and kills him. Soren and Allan then load Dichter's body in his car, planning to plant the body at the site of the raid to make it appear as though he were a bombing casualty. Insisting that Nicole must now return to England with him, Allan and Nick speed away to meet the commandos on the beach. Von Elser arrives at the house just as they drive away and pursues them. Knowing that only three minutes remain before the commandos will blow up the road leading to the beach, Allan races to beat the clock, arriving just in time to see Von Elser perish in the explosion. As Allan and Nicole walk toward the British barges that will transport them to safety, Nicole realizes that with Dichter and Von Elser both dead, she will be regarded as the widow of a German hero and therefore invaluable to the underground. After kissing Allan, she runs into the woods to hide, watching as her lover resignedly joins the troops on the barge and sails away.