A gang is scouring St. Moritz, replacing real jewels with perfect copies. At the request of the insurance companies, Lupin arrives on the scene disguised as Raoul d'Andrézy, with a stolen piece of jewelry that will enable him to infiltrate the gang. Seeing that all guests at the palace where he is staying are being searched, Lupin slips the jewel into the pocket of Marie Bonatti, a young journalist, who leaves the hotel unchecked.