Sir Edward Dominey is found half dead in the jungles of Dakar, and is brought into the camp of Baron Leopold von Ragenstein. The two men are doubles who have known each other since their school days at Eton. As Dominey heals from his safari misadventure, Ragenstein learns that England is about to declare war on Germany. When the clock strikes midnight, Ragenstein tells the alcoholic Dominey that their two countries are now at war. Later, Ragenstein tells his confidant, Dr. Schmidt, that Dominey must be killed, as he plans to go to London and impersonate his former schoolmate. After many months, Ragenstein arrives in Berlin and the German high command agrees to support his plan to infiltrate British high society as Dominey. He is then met in London by German spy Frederick Seamon, a naturalized Englishman. Ragenstein is told that his first mission is to uncover the British plans for the fortification of the English Channel, and that he is to do this through his connection to Dominey's wife's uncle, Sir Ronald Clayfair, the British Minister of Home Defense. Later, Lady Muriel Ingram Dominey, Edward's wife, lunches with Captain Franchot Bardinet of the French consulate at the same restaurant that Ragenstein meets with Mangan, the Dominey family lawyer. Muriel sees her "husband," and the two are soon joined by her uncle and aunt, Sir Ronald and Lady Leslie Clayfair. Posing as Dominey, Ragenstein tells them that he has reformed, so the Clayfairs invite their nephew to dinner that night. Ragenstein's good fortune is interrupted, however, when he is recognized by his old girl friend, Baroness Stephanie Idenbraum. At dinner, Ronald offers Ragenstein a position in Home Defense, and Muriel questions her husband about his relationship with Stephanie. Ragenstein claims that Stephanie mistook him for someone else, and Bardinet points out that the wealthy Baroness' husband was killed in a duel years earlier by her unknown lover. After Ragenstein leaves, Muriel tells her uncle that she cannot believe that her estranged husband is the same man she left. Meanwhile, Bardinet, who is actually a German spy, meets with Seamon, who tells him to end his romantic relationship with Muriel so that Ragenstein can more easily infiltrate the Clayfairs. Ragenstein then begins romancing his "wife," but the jealous Bardinet tells Muriel that he saw Dominey with Stephanie and questions whether he truly is her husband. Muriel goes to her uncle, who orders her to continue her relationship with Dominey until they can determine his true identity. Ragenstein is later questioned by intelligence officers Sir Henry Tristram and Yardly about his relationship with Stephanie, after which he is arrested as Ragenstein. Ragenstein then convinces Sir Ronald that he is indeed Dominey by recalling old family antidotes. Dominey then states that he killed the real Ragenstein and that he has convinced Seamon that there is a large faction of English high society that supports Hitler. As a result of Dominey's claims, the Germans plan to send a high-level emissary to his Scottish shooting lodge. Back at the German spies' hideout, Dominey delivers the fortification plans to Seamon and is ordered to take a U-boat back to Germany. After he leaves, both Stephanie and Bardinet question whether "Ragenstein" is himself, but Seamon dismisses their notions as jealousy. In Berlin, Dominey, still in the guise of Ragenstein, is congratulated on his work and is told that Rudolph Hess is being sent to the Dominey estate to meet with the English conspirators. After Hess's arrest is mistakenly made public, Stephanie calls Seamon and Bardinet to tell them that she is about to be arrested and warns them that real Dominey has fooled them. They are arrested as well, but are first able to shortwave Berlin about Hess's arrest. Dominey's arrest is immediately ordered, but he escapes the Gestapo and manages to sneak across the German boarder into France. He arrives back in London during a German blitzkrieg, and there he is finally reunited with Muriel.