M is in the Assize Court, waiting to testify in the trial of Gaston Meurant, picture framer, accused of killing his aunt, Léontine Faverges, for her money, and a 4-year-old girl, Cécile Perrin, who she was taking care of. The evidence had been inconclusive, and he was not a suspect until an anonymous phone call told of his blue suit in his closet, which was bloodstained. At the trial M testifies that he'd continued the investigation "unofficially" as he hadn't been convinced of Meurant's guilt. They'd found that Ginette Meurant, his wife, had been seen at a hotel with a short, heavy-set man she'd called Pierrot. As the other evidence was unclear, Meurant was found not guilty and released.
M had both him and his wife followed. She soon moved into a hotel, and he wandered the streets, eventually coming to see M, not believing the stories about his wife. M told him more, including that his brother Alfred Meurant had been with her, and that she'd had numerous boyfriends. Meurant goes to Toulon to look for his brother, whom he finds in the Eucalyptus bar, with other shady characters. Meurant talks to him for a while, then returns to Paris, followed all the while, and heads towards Chelles. Lapointe follows him there, where he sees him shoot a man, Pierre Millard, apparently his wife's accomplice. Alfred had given him a gun and directions to the man's location, apparently his grandmother's duck and goose farm. He appears to look at M with gratitude when arrested. M had known he'd had a gun, but neglected to tell Lapointe, who'd been following him.