While winding up the case of Grégoire Brau, the Canon, a burglar, a call comes in of a murder in the house of Lachaume's Biscuits, Quai de la Gare, Ivry, a brand which Maigret remembers from his childhood. The elder brother, Léonard Lachaume had been shot. The other members of the household claim to know nothing, have heard nothing. The Examining Magistrate, Angelot, one of the new breed, insists on staying for the interrogation, and taking charge of the case. The family is in dire straits, and Maigret learns that they are being supported by the money of the Armand Lachaume's wife, Paulette Lachaume, the daughter of the late, nearby Frédéric Zuberski, a wealthy leather merchant. It appears to have been a robbery, but the attempt at making it look so is clumsy and unconvincing. Someone on a barge docked nearby had seen someone breaking glass on a wall hours before the "break-in". A woman had seen a red sportscar parked nearby, waiting for Paulette.
Maigret visits the "black sheep" sister, Véronique Lachaume, and they learn, to her dismay, that her boyfriend, Jacques Sainval, was also Paulette's boyfriend, and in fact that they'd been considering marriage. At this point Angelot insists on taking over, and an interrogation of Paulette is held, Angelot using Maigret's notes. She'd been planning a divorce, and the panic-stricken family, having found out by intercepting a letter from her lawyer, has decided to do away with her. She suspects, and stays awake, killing her brother-in-law when he comes in to bludgeon her in bed, with the gun she'd gotten from Jacques. The family convinced her to go along with the cover-up. Armand, outside the magistrate's door has heard all, and kills himself with the same gun.
This report prepared by Dana Samson