In November 1941, American radio correspondent Bill Roberts sends out valuable espionage information during his broadcasts from Berlin despite the vigilance of Nazi censors. Realizing that someone must be giving Bill the information, Gestapo colonel Carl von Rau details various agents to follow the American, but Bill eludes them all. Von Rau then assigns his fiancée, Karen Hauen, to investigate Bill, even though she has never done spy work before and is reluctant to do it. Karen succeeds in getting Bill to ask her to his apartment for dinner and there discovers that the information he receives is written in invisible ink on the stamps he buys from a local shop. Karen reports her findings to von Rau the next day, but when she returns to her apartment, her father Rudolf confesses to her that he is the one who has been helping Bill. He further states that she has been inadvertantly supplying the information he has passed on to Bill. Hauen explains that he did it because he hates the Nazis, and then, in order to protect Karen, he turns himself in to von Rau, claiming that she arrested him. Although Von Rau promises Karen that no undue harm will come to the old man, Hauen is tortured. Bill is not arrested, for von Rau believes that he no longer poses a threat, and when Karen learns that her father is being sent to the notorious Grundorf insane asylum, from which no one returns, she turns to Bill for help. Bill castigates Karen for working for the Nazis, but agrees to help for Hauen's sake. By posing as a psychiatrist on Hitler's personal staff, Bill infiltrates the asylum. Bill helps Hauen escape, then gives him his own altered passport. Hauen reaches safety in Switzerland, and when von Rau discovers the subterfuge, Bill naïvely believes that nothing can be done to him because he is an American citizen. Von Rau arrests Bill, however, and on 7 Dec 1941, replaces him on his broadcast with a German impersonator, who asserts that the United States is to blame for Pearl Harbor. Bill is then sent to a concentration camp, much to the horror of Karen, who has fallen in love with him. When Karen pleads with von Rau to free Bill, he agrees to do so if she will marry him immediately. Von Rau pretends to arrange Bill's escape, but instead orders a guard to kill him. Carla Hoening, von Rau's secretary who is in love with him, tells Karen about his treachery so that Karen will try to escape with Bill. While Carla has von Rau detained, Bill escapes from the camp with Karen's aid, and they hijack von Rau's private plane. As they are flying toward Switzerland, the pilot reveals that he is glad to help them, for he also wants to leave Germany, and the relieved couple embrace.