John Martin attempts to steal plans for an electric circuit from a plant in Baltimore, Maryland. Caught and arrested for espionage, he is turned over to the Office of Strategic Services, which is training a group of new recruits by sending them on test missions.
Along with three other men, Gates, Parker and Bernay, and one woman, Elaine Dupree, Martin (an assumed name like the others) is sent to France to blow up a railroad tunnel in order to paralyze Axis troops during the Allied invasion. Martin doubts Elaine can work under pressure because she is a woman, but she insists he treat her as any male agent.
During the group's first assignment, at a rural French inn, German agents kill Gates. A meeting of the French maquis is interrupted by German Colonel Paul Meister, who becomes immediately infatuated with Elaine. Elaine sculpts a bust of Meister's head, and when he announces his departure for Normandy on a troop train, she begs him to take her with him. With Martin's help, Elaine makes a duplicate bust and fills it with plastic explosives. She and Martin then blow up the tunnel. After Martin comes back for her under fire, Elaine makes him promise never to jeopardize a mission in order to rescue her.
On foot, they meet Bernay, who is their radio contact with the O.S.S. As the Allies break through at Normandy, Elaine and Martin make a deal with Amadeus Brink, an officer of the Gestapo, who hopes to secure his safety and a small fortune. Brink removes Martin and Elaine's "wanted" file from the Gestapo sector headquarters and arranges for his cousin, a courier, to hand over a diplomatic pouch to Bernay for photocopying. Bernay places the negative in the lining of Martin's hat. Against Brink's advice, Bernay radios a final message that the Nazis have broken one of the O.S.S. secret codes. Bernay is gunned down, and Martin and Elaine are questioned by the Gestapo, but are released before Meister catches Brink with their file papers.
Martin and Elaine are about to board a plane to safety when they are asked to complete one last mission. They must contact Parker, another agent who is on assignment near the Rhine, and Martin being the only agent he'll recognize. At a farmhouse, Elaine is accosted by a group of drunken German soldiers. Parker, hiding among them, gives Elaine the Germans' troop dispositions. Martin leaves the farmhouse to radio in the positions. While he is gone, Meister arrests Elaine.
As the American troops advance through France, Cmdr. Brady tells Martin that Elaine's real name was Ellen Rogers, and he imagines that she might have been the girl next door in his hometown.
