After Neville Chamberlain announces the signing of the Munich Pact with Germany, RAF officer Keith Wilson becomes disgusted with Britain's appeasement and denounces his country's foreign policy. In protest, Keith paints swastikas on the Nelson Column and is court-martialed and discharged from the service for his actions. Soon after his public humiliation, Keith is summoned to Scotland Yard by Colonel Patterson, who enlists him in the counterespionage division of the British Secret Service, instructing him to pose as a British traitor and infiltrate the Nazi propaganda bureau. Patterson directs Keith to attend a party and, there he meets Ilse Von Preissing and her brother Rudolph, a prominent member of the Nazi propaganda bureau. To accomplish his mission, Keith cultivates a romance with Ilse, and after she and Rudolph leave for Antibes, he follows her there. After losing heavily at the gaming tables, Keith is confronted by the proprietor, Henri Rader, who demands payment. Unable to pay his debts, Keith agrees to photograph a model of an aerial torpedo in London, which the Nazis are anxious to see. By arrangement with Patterson, Keith photographs the model and is apprehended by Scotland Yard while handing the film over to a Nazi agent. For his crime, Keith is sentenced to eighteen months in prison. By the time he is released, war has been declared, and he is recruited by Rudolph to come to Berlin and advocate British surrender on a German radio program broadcast to Britain called "The Voice of Truth." When American correspondents Bill Banning and Babe Forrester, who are stationed in Germany, hear Keith's broadcast, they denounce him as a traitor. Keith's efforts are applauded, however, by Gretta Van Leyden, a Nazi propagandist. One day, Gretta visits Keith at his office and asks him to critique a forthcoming article she has written. In reality, Gretta is also a counter-espionage agent and her speech contains coded information about Nazi installations for him to transmit to England. One day, Keith visits a florist named Hoffman, whose shop is on the outskirts of town. Ilse, passing the shop, sees Hoffman pin a boutonniere on Keith, and later Keith extracts a secret message from the flower, detailing the damage that British bombers have wrought on Germany. When he returns to the office, Ilse, who has begun to question Germany's brutal tactics, calls Keith a traitor to his country and asks him about his visit to the flower shop. Their argument is interrupted when Gretta, disguised as a nurse, phones Keith and instructs him to meet her at the train station. There, Gretta informs Keith of Hoffman's arrest and her imminent danger. After passing him a vital message about Germany's planned invasion of England, Gretta sees the Gestapo enter the station. Realizing that they have come to arrest her, Gretta tells Keith that the message is more important than her life and that to maintain his cover, he must expose her to the Nazis. Forrester and Smitty, another correspondent, witness Keith's betrayal of Gretta, but before the Nazis can arrest her, she swallows a poison capsule and dies. Despite Gretta's sacrifice, the Nazis are suspicious of Keith and phone Rudolph, ordering him to cancel the broadcast and arrest Keith. Overhearing the conversation, Keith strikes Rudolph on the head with his gun, rendering him unconscious. Turning to Ilse for help, Keith tells her he must reach an underground radio station in Holland to transmit the critical warning. When Ilse responds that Rudolph was also on his way to Holland, Keith realizes that the Nazis are planning to launch their invasion from there. Soon after, Forrester and Smitty arrive at the office, demanding to see Rudolph, and Keith reveals that he is a British agent and asks their help in decoying the guards until he can escape. Dressed in Rudolph's uniform, Keith speeds away in the Nazi's car, driven by Ilse. Upon regaining consciousness, Rudolph alerts the Gestapo to Keith's deception and orders that he and Ilse be shot on sight. After escaping to Holland, Keith and Ilse locate the farm that houses the underground. Meanwhile, Rudolph takes flight to find the fugitives. As the sound of Rudolph's plane engines rumble overhead, the Gestapo approach the farmhouse. When one of the members of the underground questions his allegiance, Keith offers to surrender to the Gestapo if they will allow Ilse to broadcast the crucial message. To save Keith, Ilse runs outside and is gunned down by her own brother. After relaying the message that the Nazis plan to launch their attack from the Frisian Islands, Keith runs from the farmhouse to notify the others. The agents are in the midst of broadcasting the warning to Britain when the Gestapo bursts into the house and kills them all, truncating the transmission. Keith commandeers Rudolph's plane, then flies to the coast and radios Britain that the invasion fleet is anchored off the Frisian Islands. When the Nazis gun down his plane, Keith crashes his craft into the islands, creating a firey target for the oncoming British bombers. For his act of heroism, Keith is posthumously awarded the Victoria Cross.