Maigret had just wound up the case of what the papers have called the Motorcycle Gang, young men on motorcycles robbing jewelers. In the morning Lapointe came by to tell him Lognon had been shot, in the Avenue Junot, and taken to Bichat for surgery. At the Avenue Junot they find that Lognon had been staying nights in the room of Marinette Augier, but it soons becomes clear that he was doing some kind of surveillance. Marinette Augier has disappeared since the shooting. No one knows what he was working on, except that it was "something" big. The inspectors of the 18th are handling the case, and Chinquier finds an old man, Maclet, who spends all day in front of his window, and has seen many young women coming in and out of the Dutchman's, Norris Jonker's, a rich art collector across the way.
Though Chinquier had gone there, Maigret goes to interview him, and receives a hostile tour of the house, culminating in the studio of Jonker's young wife, Mirella Jonker, from Nice. He finds a strange room with an iron bed, a lock on the outside, and graffiti on the walls. Jonker's explanations seem insufficient. Checking in Nice and London, he learns that Mirella was formerly Marcelle Maillant, had been involved with a shady character, Stanley Hobson. She'd kept up with him after marrying an Englishman, Herbert Muir, which had led to their divorce.
A door-to-door salesman, Langeron, had witnessed a man being taken into a yellow Jaguar in front of the house, and it is traced to Ed Gollan, an art critic staying at the Ritz. Marinette Augier has been found by Lagrume, who brings her back to Paris. She'd been afraid and gone into hiding, fearing Lognon's attempted killers would look for her too. Maigret gathers them together at the Quai des Orfèvres. Norris Jonker had been involved in art forgeries, produced by a volatile young artist, Federigo Palestri, with a passion for painting and young girls, whom he'd kept in the studio room. Lognon had been noticed and the artist sent away. He'd been killed by Mario de Lucia, or perhaps hanged himself. Lucia had made a run for the Belgian border but been caught. Jonker was sentenced to a year, Mirella got off, and the others were sentenced for terms up to ten years... Lognon recovered and went off for a two month recuperation with his wife.
The review of this Book prepared by Dana Samson