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During World War I. British Intelligence is baffled by the clever German spy Franz Shindler, and dispatches pilot Frank Bennett to pick up Williams, a crack British spy operating behind German lines. Before he can complete his mission, Frank's plane is shot down and he is hospitalized in France, where he meets and falls in love with his nurse, Helene von Lerbeer. Helene, a German spy, abruptly leaves Frank when German Intelligence places her in the London home of Frank's father Arthur Bennett, a new cabinet minister. Posing as Frances Hautry, a fugitive from the German detention camps, Helene is offered sanctuary at the Bennett house and there she meets Valdar, a German agent posing as a servant. Valdar tells Helene that he is her liaison to Shindler, who is now operating in London. Helene falls under suspicion when Crichton, a British agent, discovers Bennett's secretary sending messages to her in German code. Allegiances become more complicated when Valdar, sans his French accent, visits Colonel John Yeats, the head of British Intelligence, and reveals himself as a double agent working undercover for Britain. Yeats confides to Valdar that a British cabinet meeting is to convene secretly the next night in the Bennett library. Meanwhile, to trap Frances, Yeats has a British agent impersonate Kurtz, an escaped German spy, but Frances outwits Yeats and turns the fake Kurtz over to the police. Frances warns Valdar that she has been discovered, but he insists that she remain at the Bennett house until the cabinet meeting has convened. Shortly after, Frank arrives home from France and recognizes Helene, who confesses that she is a British agent. Overhearing her confession, Valdar imprisons her in the basement, where he reveals that he is Shindler and accuses her of being a British spy. After Helene convinces Valdar that she is loyal to Germany, he shows her the bomb that he has set to destroy the cabinet ministers. At that moment, Yeats and his men pound at the basement door, and Helene lets them in to defuse the bomb. Shindler escapes through a coal shoot, but meets his death in a German air raid.