An FBI agent risks his life to infiltrate Nazi sympathizers in the U.S.

In a small Scottish town in the late 1930's, Mrs. Mary MacLaughlin operates a secret international Nazi postal office out of her home. Her services are provided to agents working all over the world, including Kurt Schneider, an American soldier living in New York. Dr. Karl F. Kassell, a U.S. Navy Reserve officer, also works for the Nazis--he heads the New York German Bund and is devoted to the "purification" of the German race. Schneider's career as a spy begins with orders to report to the Nazis on the number of American troops stationed in the New York area. He carries this task out successfully, but complains when he is paid by the Nazis a meager monthly wage of fifty dollars. Meanwhile, at a New York Bund rally, Gestapo agents forcibly remove a dissenting voice from the meeting. Subversive Nazi activities are also taking place on board the German ocean liner Europa , where Franz Schlager is the ship's Nazi leader. Schlager works closely with the ship's beauty salon operator, Hilda Keinhauer, who reports passenger Anna Keller when she learns that Keller does not sympathize with the Nazi regime. Upon his arrival in New York, Schlager is instructed to make contact with Schneider and give him a new assignment. Impatient for better work, Schneider sends a letter to Nazi officials in Germany, but when the letter is intercepted in Scotland, Mrs. MacLaughlin is arrested. The evidence found in MacLaughlin's home prompts an F.B.I. investigation, led by Edward Renard, into Nazi espionage activities in the United States. Federal agents are soon tipped off to one of Schneider's assignments and arrest the Nazi operative. Schneider is brought to Renard for questioning, and Renard cleverly extracts a full confession from him. When Hilda Keinhauer, whom Schneider implicates, is arrested, she unintentionally implicates Kassell. Renard surprises Kassell at his office, and he, too, eventually cracks under pressure, naming others involved in the spy ring. A nationwide dragnet is ordered, and many more agents are arrested, including Hintze and Wildebrandt, who are later released. Although Renard tries to protect Kassell from Hintze and Wildebrandt, he is too late to prevent them from abducting him and forcing him to board the German liner S. S. Bismarck for Germany. Renard sends orders for the ship to stop and surrender Kassell, but the captain refuses to obey them. When the ship docks in Germany, the Gestapo orders Kassell to file formal charges of harassment and intimidation against the F.B.I. Meanwhile, Dr. Julius Gustav Krogman, a German government official, appears at Renard's office to advise Keinhauer to lie and say that she was forced to sign a false confession. Renard dismisses Krogman from his office, and realizes that the official's attempt to intercede on Keinhauer's behalf proves the German government's complicity in the espionage crimes. The spy case goes before a grand jury, and four of the major participants in the spy ring are convicted and sentenced.