During World War I, a master German spy named Valdar, Boris Karloff, has cost the British dearly. In desperation, they dispatch their best agent, who is currently undercover in Germany. Pilot Frank Bennett, Bruce Lester, is sent to bring him home, but the Germans are forewarned and Bennett is shot down. He survives the crash and is rescued by friendly soldiers. While convalescing in a hospital, Bennett is tended by a pretty nurse, Helene Von Lorbeer, Margaret Lindsay. He confesses his love, but she explains that she is leaving and that they will not meet again. Yet, after he drifts to sleep, she steals a tentative kiss on his cheek.
Helene turns out to be a spy herself. She is recalled to Germany to receive a high honor personally from the Kaiser and to begin a fresh mission. Posing as a refugee named Frances Hawtrey, she infiltrates the London household of Arthur Bennett, a cabinet minister and, coincidentally, Frank’s father. She takes her orders from Valdar, the household’s seemingly loyal butler. Valdar, in turn, secretly reports to Colonel Yeats, the head of British Intelligence.
When Bennett’s secretary—the officious Miss Risdon, Louise Brien—taps out a coded message on her trusty typewriter, Yeats is present and recognizes the signal. Since Hawtrey is the only other person who could explain the message, Yeats sets a trap for her. A captured spy named Kurz seems to escape from the British and flees to Hawtrey’s bedroom. She hides him in her closet, but then betrays him when Yeats and his men arrive. Afterwards, she tells Valdar that she knew “Kurz” was an imposter.
Frank Bennett unexpectedly returns, his squadron and others having been recalled to London for reasons he cannot fully grasp. He is astonished to discover his former nurse under a different name within the same house. Valdar overhears their conversation, and the tension in the Bennett home climbs.
That night, the British cabinet meets in Bennett’s residence, and it is the moment Valdar has been waiting for. He forces Hawtrey at gunpoint down into the cellar, where he has rigged a bomb to blow the house up under cover of a Zeppelin bombing raid. Hawtrey explains that she had no choice but to spin a tale to allay Frank’s suspicions. Convinced when she shows him the award she received, Valdar finally reveals that he is Strendler.
Fortunately, Valdar has been under close surveillance. Yeats and his men rush to the cellar door. When Valdar escapes through the coal chute, Hawtrey unsecures the door and informs Yeats about the bomb, proving that she is indeed a double agent. Valdar hurries to his hideout to transmit the stolen British plans for the spring offensive, hotly pursued by the British. In a cruel twist of fate, a Zeppelin bombs the location, killing Valdar and his conspirators before he can send any actionable information.
