At the Parisian home of millionaire Gourney-Martin, a daring robbery triggers a swift police response. Detective Guerchard, Lionel Barrymore, suspects Arsène Lupin, John Barrymore, as the mastermind and orders detectives to tail a getaway vehicle. Inside, the passenger is the Duke of Charmerace, who swears he has been robbed, and Gourney-Martin soon confirms the Duke’s identity, noting that his valuables are stored at the country estate. Guerchard reluctantly releases the Duke.
The next day, Guerchard examines footprint impressions outside the mansion, convinced they match Lupin’s method. The Prefect of Police hands Guerchard a letter from Lupin, who plans to attend the Duke’s hall that evening; in frustration, the Prefect orders Lupin’s arrest within a month. At the ball, Sonia, Karen Morley, a young countess who is really Sophie Krellberg, dances with the Duke while bailiffs attempt to collect past payments from him. The Duke, using a tense game of trust, has Guerchard’s credentials checked, and the party atmosphere turns tense as the lights go out and several guests have their jewelry stolen.
Believing the Duke is Lupin, Guerchard learns from Sonia that Gourney-Martin has invited them to leave for the country estate tonight. There, the Duke sneaks into Gourney-Martin’s study and finds him checking his safe; he’s warned that the safe lock can deliver an electrical shock. The next morning, a letter from Arsène Lupin threatens to steal the Mona Lisa from the Louvre at midnight if his men aren’t released. Frightened, Gourney-Martin calls for police protection, and when the expected burglary doesn’t occur, Guerchard ends up with the front key to the estate. While the Duke dines and spends the night with Sonia, Lupin’s men steal Gourney-Martin’s paintings.
The following day, Gourney-Martin discovers the theft, but Guerchard’s interrogation of Lupin’s men yields no confession. Sonia’s true identity is revealed, but Guerchard remains skeptical. A rock crashes through a window bearing another Lupin message, demanding his men’s release or the Mona Lisa will be stolen from the Louvre. At the Louvre, the real Mona Lisa is placed in the vault, but a diversion allows Lupin to open the vault and lift the painting, with Guerchard discovering that Lupin has disguised himself as Laurent, an elderly flower seller.
At the Duke’s hideout outside Paris, he schemes to escape, but Guerchard arrives to arrest him. The Duke confirms that he is Arsène Lupin and offers a pact: he will trade the Mona Lisa for the release of his men. Guerchard refuses to bargain, yet the Duke releases Maria, Guerchard’s daughter who had been kidnapped, as a gesture of leverage. On the way to police headquarters, the Duke reveals where he hid Gourney-Martin’s jewels and how he obtained Guerchard’s shoes, leaving Guerchard conflicted about the case’s outcome. Guerchard then recalls a past failure when a criminal jumped into the Seine; the Duke tries the same river jump, and Guerchard fires four shots, hearing the criminal scream.
The Prefect of Police, unmoved, refuses to accept Guerchard’s resignation. In a final twist, at a jewelry store, the Duke (who survived) and Sonia browse for gold rings, and Lupin cheekily riffs on Guerchard’s fate with the line: > wouldn't old Guerchard love to see me buying jewelry?
