During World War II, Charles Stevenson Gibson, a St. Louis attorney with an extensive background in international affairs, is chosen by President Roosevelt to organize the secret activities of a new Intelligence Corps. Gibson, in turn, selects Robert Sharkey, a widely traveled, multi-lingual scholar who served with distinction in World War I, to administer the complex training program. Selected groups of volunteers report to Washington for rigorous training before assignment overseas. In 1944, the candidates selected for the 77th group include Suzanne de Beaumont, a French citizen who became stranded in the U.S. when France fell, and whose husband is an artillery officer in the French army. Jeff Lassiter, the son of an American consul, educated in Geneva and Oxford and recruited from the Officers' Training School at the University of California at Los Angeles, and Bill O'Connell, a Rutgers graduate and former employee of the foreign department of a major bank, are also chosen. At a secluded country estate, the twenty-two candidates are given two weeks of intensive testing to see if they qualify for further training. Gibson tells Sharkey he knows that one of the candidates is a German agent and Sharkey is assigned to identify him. The group undergoes tough athletic drills, observation tests, instruction in Morse Code, interpretation of sounds, mock interrogations and hypothetical and actual missions. Lassiter and O'Connell, who are room-mates, are sent on a trial mission together. Sharkey is convinced that O'Connell is the German agent because he is too good to be a beginner. Gibson confirms Sharkey's opinion but wants O'Connell, whose real name is Wilhelm Kuncel, to go as planned to England, where he will be fed misinformation about the second front campaign. In London, Sharkey and O'Connell visit the Netherlands Section of Allied intelligence, and O'Connell absorbs false information on Plan "B," the invasion of Germany through the Lowlands. Suzanne and Lassiter, meanwhile, are assigned a mission in France, where they are to locate a Frenchman named Duclois who has built a factory in which the Germans are manufacturing V-2 rocket bombs. They plan to enter France via Holland and no one, including O'Connell, who is to accompany them, must know what their mission is. When Sharkey then tells Lassiter that O'Connell is a German agent, he is shocked, but agrees to shoot O'Connell if he threatens to defect. Just after a farewell dinner, Suzanne learns that her husband is dead. When the trio parachute into Holland, one chute fails to open and Sharkey receives a message from Suzanne that Lassiter was killed in the jump and O'Connell has disappeared. A later cable reveals that Lassiter's parachute cord was deliberately cut. Knowing that the lives of all the agents are in danger as O'Connell can identify them, Sharkey takes over Lassiter's assignment and rendezvous with Suzanne at a safe house. At Nazi headquarters, meanwhile, O'Connell tells his superiors that they must find Lassiter's replacement. Sharkey, posing as an official of the Vichy government's Department of Labor, visits the mayor of the town where the rocket bomb factory is located and demands to meet Duclois. The mayor is actually a member of the French Underground and once Sharkey has proven his true identity, agrees to help him extricate Duclois from the heavily guarded Hotel Moderne where he lives and works. Meanwhile, the German state police have been investigating Sharkey and at Nazi headquarters, at 13 Rue Madeleine in Le Havre, O'Connell identifies a sketch of the American. As part of an elaborate plan, the mayor seeks protection from the Nazis against his countrymen, who regard him as a collaborator. Needing more men to protect the mayor, the Germans withdraw those guarding Duclois, enabling Sharkey to capture him. While the Underground put Duclois on a plane, Sharkey blocks the approaching Germans by deliberately crashing a car into them and is captured. Having witnessed the crash, Suzanne attempts to send a cable to Gibson, but is discovered by the Germans and killed. Later, in London, Duclois gives details of the factory layout to the RAF, who prepare to bomb it. Gibson receives part of Suzanne's final transmission in which she states that Sharkey is a prisoner at Nazi headquarters. As the Allies intend to bomb 13 Rue Madeleine, Gibson tells the pilots that Sharkey is no doubt being subjected to severe torture and that bombing the place may be the only way to release him from his suffering. Sharkey has been tortured but has revealed nothing. The bombing starts and Sharkey dies laughing in O'Connell's face, knowing that the vital D-Day secrets will perish with him.