A serial killer had been stalking Montmartre, with five women stabbed to death, one a month. So far there are no clues, and the district is in a state of siege. Maigret arranges an interview in his office that the journalists will take for the interrogation of a suspect, and by that means Maigret hopes to goad the killer into a new attack, as if to make him say "you've got the wrong man". In preparation Maigret has organized a massive and secret infiltration of Montmartre by police and women's auxiliary forces from all over Paris to catch the criminal when he attacked. Maigret's plan almost succeeds, for a young woman, Marthe Jusserand, one of his auxiliaries, was attacked, and knowing judo, almost captured the man. He escaped, but she'd seen him, and managed to grab one of his buttons and a piece of cloth from his jacket.
By luck, the cloth was unusual enough for Maigret to soon put his hands on the likely suspect, Marcel Moncin, an interior designer, who'd grown up in Montmartre. Jusserand identifies him, but he refuses to confess. And then, the day after his arrest, there is another murder. But this time Maigret understands. It was either Moncin's fiercely possessive mother, who still lives in Montmartre, or his wife, Yvonne Moncin. He brings the three to his office for a final confrontation, and though the mother confesses, to show her great devotion to Moncin, it was in fact Yvonne who'd murdered a girl to put the police off the track. Maigret says that while Moncin will no doubt spend his life in custody in some hospital, his wife will be regarded as a cold-blooded murderer of an innocent girl.
This report prepared by Dana Samson