In London in 1944, prior to the Allied invasion at Normandy, Capt. Bill Ranson, a United States agent attached to the Office of Strategic Services, is assigned to work with a British intelligence unit headed by Major Howard. Posing as documentary filmmakers from Arista Films, the unit's mission is to make the Germans believe that the Allies will launch their invasion in Holland, thus assuring that the Germans will deploy their troops to Holland, leaving the Normandy site unguarded. Among the Dutch agents working with Howard are Prof. Mulder, a Jew whose wife has been killed by the Germans, Sgt. Parrish, Piet and Jan, the radio operator who is scheduled to fly back to Holland. Only Ranson and Howard have been informed of the group's directive, however. When Ranson accompanies Jan to the airstrip where he is to meet Rolande Hertog, Jan's replacement, he is surprised to discover that Rolande is a very attractive female agent. At the office of Arista Films, Howard unveils a map of Holland and points to the area in which the Allies are to land. Soon after, word comes that Jan has been captured by the Germans. Later, when Ranson questions Howard about his tactics, Howard becomes irritable over Ranson's lack of confidence in his judgment. That night, Ranson invites Rolande to dinner. At the tavern, Ranson sees Parrish, the agent assigned to watch the office. When Ranson asks why Parrish is at the tavern and not the office, Parrish replies that Howard gave him permission to leave. Concerned, Ranson and Rolande drive back to the office and find the door unlocked. Ranson enters, and when one of the intruders inside fires a shot, Ranson shoots him, hands Rolande a gun and instructs her to guard the man while he searches the office. In Ranson's absence, the intruder starts to speak to Rolande in German, causing her to shoot him in cold blood. When Ranson returns, Rolande, feigning hysterics, claims that she shot in self-defense. To calm Rolande, Ranson holds her and after they kiss, he escorts her back to her apartment. Later that night, Ranson is called to the office by Howard, who angrily informs him that he knew the Germans planned a raid and therefore deliberately arranged for the office to be deserted so that the agents would find misleading documents concerning the invasion. Howard then confides that he suspects Rolande of being a German spy, but instructs Ranson to continue his courtship so as not to arouse her suspicions. Soon after, Piet is sent on a perilous reconnaissance mission to Germany, and after he leaves, word comes that he has been captured. Despite Ranson's efforts, Rolande senses a change in his ardor, realizes that he has penetrated her cover, and begins to suspect that the Holland invasion is a ruse. Rolande notifies her contact, Dr. Radamacher, a German spy posing as a dentist, to inform him of her conclusions, but he is skeptical and demands evidence. To obtain proof, they decide to kidnap Prof. Mulder's young son Willem and then threaten to kill the boy unless Mulder cooperates. Desperate, an unwary Mulder seeks out Rolande to ask for help. After telling Rolande that the Germans are holding Willem until the Holland invasion, Mulder confides that he fears Holland is not the target and Willem will be killed. Rolande hurries to tell her co-conspirators that the invasion will not occur in Holland, and when they refuse to believe her, she angrily notifies Scotland Yard about Willem's kidnapping. Mulder, meanwhile, relates his story to Howard and Ranson, who then join forces with Scotland Yard to find the boy. Under the pretense that a bomb has been planted in the building, the police break into Radamacher's office and free the boy. Trapped, Radamacher swallows a lethal cyanide pill, but his confederate, Martins, escapes. Ranson then hurries to Rolande's apartment and finds her hiding in the cellar, transmitting a message. Realizing that she has uncovered the truth, Ranson bluffs her by declaring that the mission has been an elaborate ruse to trick her into believing that the troops are not heading for Holland so that the Germans will deploy their troops elsewhere. To convince Rolande, Ranson allows her to grab a gun and shoot him. As he lies wounded on the floor, he listens with satisfaction as she transmits word of the Holland invasion. Rolande then flees the cellar and takes refuge in the shadows of an underground tunnel, where Martins shoots and kills her.