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Just after a robbery has been reported at the home of Parisian millionaire Gourney-Martin, police detective Guerchard and his men chase a fleeing car, only to find the passenger tied up and claiming to have been robbed. Guerchard is convinced that the man is notorious jewel thief Arséne Lupin, although he claims to be the Duke of Charmerace. Gourney-Martin later confirms the duke's identity, then laughs because all his valuables are safely hidden at his country estate. The next day Guerchard is perplexed to discover that footprint impressions of the thief found outside Gourney-Martin's home, actually came from his own shoes. He then receives a note from Lupin saying that he will be at the duke's ball that night, so Guerchard decides to go himself. At the party, the duke, who really is Lupin, is confronted first by bailiffs supposed to collect past due bills, then by a beautiful woman, Sonia, Countess Krichnoff, lying in his bed. Later, when he has a birthday cake brought into the ballroom while the lights are out, many of the guests are robbed and Guerchard is certain that the duke is indeed Lupin. Later, Gourney-Martin invites Lupin and Sonia to his country estate for the weekend. Sonia, whose real identity is petty thief Sophie Krellberg, is secretly working for Guerchard to obtain her release from prison. Late that night, at Gourney-Martin's estate, Lupin goes to his host's study and watches as Gourney-Martin demonstrates the electrical shock feature of his burglar-proof safe. As Lupin returns to his room, he finds Sonia, who pretends to sleep walk, and he pours water on her. The next morning, she finds that her fake diamond bracelet has been replaced by a real one, then Gourney-Martin receives a note signed "Arséne Lupin" threatening to steal everything from him because he was a war profiteer. That night, Lupin pretends to sleep walk into Sonia's room, but instead of throwing him out she warns him to stay upstairs as she knows Guerchard is in the house. The next morning, Gourney-Martin discovers that his safe has been robbed. Although some of Lupin's cohorts are captured, none will name him, despite Guerchard's entreaties and threats, and even Sonia stands by her statement that "the duke" spent the entire night in her bedroom. Just as Guerchard threatens to return Sonia to prison, a rock comes through the window bearing a note from Lupin saying that he will steal The Mona Lisa from the Louvre Museum the next day. During the confusion, Lupin and Sonia drive off, then he goes to a hideout outside of Paris to plan the robbery. The next day, despite the precautions of Guerchard, the painting is stolen when Lupin's careful planning results in a series of diversions. He is able to get away because he has changed identities with an old flower vendor named Laurent. Though he is frustrated by the theft, Guerchard finally realizes that he can capture his nemesis by following Laurent's horse to Lupin's hideout. As Guerchard approaches, Lupin handcuffs himself to Sonia and tells the detective that she captured him. He then offers to exchange the painting for the release of Guerchard's daughter Maria, who, unknown to her father, has been kidnapped. Though Guerchard makes no deal, Lupin releases Maria and goes with the detective. On the drive to headquarters, Guerchard mentions that he once lost a prisoner who lept into the Seine, and Lupin quickly makes a break for the river. Later, Guerchard tells the Prefect of police that he fired four shots and heard Lupin scream. In a jewelry store, as Lupin and Sonia shop for gold rings, Lupin jokingly says "wouldn't old Guerchard love to see me buying jewelry?"