On 8 December 1941, the day after the Japanese attack on Pearl Harbor, New York City hotel owner Henry Bremmer, anticipating that the Germans will declare war on the United States, begs his lawyer and friend, Carl Becker, to use his influence to get Henry's wife out of Germany. While Carl travels to Washington, D.C. on Henry's behalf, Fred, Henry's chauffeur, introduces Henry to Dr. Woodford, a Nazi spy. Woodford offers to help Henry if he will agree to work for the Nazis. At first, Henry refuses, but when Carl is unable to help him, Henry gives in to Woodford's demands. When Jim Jackson, a secret service agent on Woodford's trail and Carl's friend, arrives in New York, Carl suggests that he stay in Henry's hotel. Jim is recognized by other employees working with the Germans, however, and is murdered with poisoned gas. After Jim's body is discovered by his fellow agent, John Trent, the death is ruled a suicide, but Trent and Carl are both convinced that Jim was murdered. In order to avenge Jim's death, Carl joins the secret service. Trent is impressed by Carl's dedication and when the opportunity arises to go after Woodford, Trent and Carl head the investigation. Carl tells both Henry and Paula Fengler, the singer who is his fiancée, that he intends to find the men responsible for Jim's death. The Germans then order Henry to leave for his hunting lodge in the Adirondacks so that he cannot warn Carl. Later, the spies try to kill Carl. He pretends to be dead and, with Trent's help, overcomes the killers. Not realizing that Paula is also an enemy agent, Carl tells her that the arrested men are on a train heading for Washington, D.C., and she arranges for their escape. Thanks to a clue left behind by Henry, Carl realizes that the escaped men are hiding at the lodge. The secret service surround the lodge and a gunfight ensues. During the shooting, Henry sneaks into the radio room and is killed when he signals the location of a German U-boat. Woodford escapes again and is followed by Carl to Paula's apartment. Carl kills Woodford and Paula pretends that she was forced into hiding him, but Carl reveals that he knows Paula was married to Woodford, whose real name is Otto Zimmer. When Carl and Trent learn that a U-boat was destroyed because of Henry's message, they realize that he has always been loyal to the United States.