Following yet another daring robbery, the Minister and the Prefect of Police call on a policeman unknown to Lupin, stationed in Africa: Victor offers to play the part of a clever worldly burglar, Mister Lorchester. In this way, he becomes part of Lupin's gang, whose only task is to arrest him. But to make him look the part, Victor needs a well-funded bank account, which the enthusiastic Préfet immediately opens for him. During his evenings of idleness, Victor Lorchester meets a young woman he believes to be Lupin's friend.

During a reception at the home of the brilliant young Maxime Bermond, the guests learn that banker Dautrec has just purchased the priceless royal diamond. Back at home, Dautrec checks the locking systems on the doors and windows. A nun is waiting to read to him. Shortly afterwards, an alarm goes off in the house. The servant finds his master murdered.

Lupin, posing as Bernard d'Andrezy, visits Gournay-Martin, the wealthy champagne owner. A reception is held, during which the guests tour the cellars. During the tour, Andrezy makes strange chalk marks on the walls. In the evening, a telegram upsets those present: “Arsène Lupin is among you”. The Prefect of Police takes an immediate interest in the case, and one of the guests accuses Andrezy of being Lupin, having noticed his strange behavior in the cellars. A manhunt begins, and Lupin ends up trapped in a cul-de-sac. His old enemy, Commissaire Guerchard, has the pleasure of handcuffing the notorious burglar for the first time.

Inspector Bechoux suspects Barnett, a private detective in whom he believes he recognizes Arsène Lupin, of stealing “King Dagobert's treasure” and hiding it. Barnett goes to the church in a small village where Inspector Bechoux has begun his investigation. The theft was committed in the middle of the night by a one-armed man. So Bechoux arrests a one-armed man in the village, interrogates him and hands him over to Baron de Gravières, the local Châtelain, for safekeeping. But Barnett isn't happy with this hasty arrest.