A master German spy named Franz Strendler has cost the British dearly. In desperation, they send for their best agent, currently undercover in Germany. Pilot Frank Bennett (Bruce Lester) is sent to pick him up, but the Germans are forewarned and Bennett is shot down. Luckily, he survives and rescued by friendly soldiers. While he recovers in a hospital, he is tended by a pretty nurse, Helene Von Lorbeer (Margaret Lindsay). However, she is herself a spy. She is soon recalled to Germany to receive a high honor and undertake a new mission. Posing as a refugee named Frances Hautry, she infiltrates the London household of Arthur Bennett (Holmes Herbert), an important government official and coincidentally Frank's father. She takes her orders from Valdar (Boris Karloff), the butler. However, unbeknownst to her, he is a British double agent. He later secretly reports to Colonel Yeats (Leonard Mudie), the head of British Intelligence. When Bennett's secretary taps out a secret message in code on her typewriter, Yeats recognizes it. Since only Hautry is also in the office at the time, he sets a trap for her. A captured spy named Kurz seemingly escapes from the British and flees to Hautry's bedroom. She hides him in her closet, but then betrays him when Yeats and his men show up. Afterwards, she tells Valdar that she knew "Kurz" was an imposter. When Frank Bennett unexpectedly shows up on leave, he is surprised to find his former nurse there and under a different name. Hautry is forced to reveal that she is loyal to the British. However, Valdar overhears their conversation. That night, the British cabinet meets in Bennett's home. It is the moment Valdar has been waiting for. He forces Hautry at gunpoint down in the cellar, where he has a bomb set to blow the house up. Hautry tells Valdar that she had no choice but to make up a story to allay Frank's suspicions and is loyal to Germany. Convinced when she shows him the award she was given, Valdar finally reveals that he is Strendler. Fortunately, Valdar has been under surveillance. Yeats and his men rush to the cellar door. When Valdar escapes through the coal shute, Hautry reveals her true allegiance by unlocking the door and informing Yeats about the bomb. Valdar rushes to his hideout to transmit the stolen British plans for the spring offensive, but ironically a German Zeppelin air raid bombs the place and kills him and his confederates.