Based on a play by LeBlanc and de Croisset, a notorious jewel thief is finally caught by a determined police detective.
After a robbery the police arrest the Duke of Charmerace (John Barrymore); but the owner identifies him and says nothing was stolen; so he is released. Detective Guerchard (Lionel Barrymore) examines the footprint evidence and gets a note from Arsene Lupin that he will be at Charmerace's. Charmerace puts off two bill collectors and finds Sonia (Karen Morley) naked in his bed. Two men mistakenly arrest Guerchard, and Charmerace questions him. While the lights are out when a cake is brought in, three women say they had jewels stolen. Guerchard conducts a search, and Sonia reports to him.
Charmerace follows Sonia to the mansion of Gourney-Martin, who shows Charmerace his shocking safe. Sonia reports jewelry she received with a note from Arsene Lupin. Gourney-Martin gets a note his jewels will be taken because of his war profits and the misery he has caused. Sonia is afraid Charmerace will be taken as Arsene Lupin. Police remove art objects. Gourney-Martin calls the police, who arrive without their uniforms; but a wagon unloads the thieves in the police uniforms. Gourney-Martin find his safe empty. Guerchard questions the thieves; but none talk, and Sonia says Charmerace was with her all night. Guerchard says Sonia is a swindler on parole and orders her back to prison. A note says Arsene Lupin plans to steal the Mona Lisa.
Charmerace disguises himself as the double of an old man, and gets away with the real Mona Lisa stolen from the vault after a panic over fire. Guerchard questions the old man, and Charmerace brings the painting to Sonia. Guerchard has his place surrounded; so Charmerace puts on handcuffs so that Sonia can go free. Charmerace trades the freedom of six men for the painting and tries to trade Guerchard's kidnapped daughter for himself; but Guerchard refuses. Charmerace releases the daughter; but on the way to the station Guerchard allows Charmerace to escape in the river. Guerchard resigns, and in the final scene Charmerace buys a gold ring for Sonia.
Detective fans will enjoy the many twists in this elegantly acted drama. Not only does Charmerace get away with stealing, he doesn't even pay his bills. As clever as he is, he cannot manage to continually outwit the long arms of the law. Sonia's fear of prison for either of them and her love persuades Charmerace to actually buy a ring. Perhaps both can reform, as Guerchard enjoys his retirement.