Maigret has been assigned to Luçon, in the Vendée, as a result of political problems in his department. He receives a call from Adine Hulot, reporting that she has seen a corpse on the floor of a bedroom at Judge Forlacroix's house in L'Aiguillon. Maigret goes there, a small village of mussel gatherers, and interrupts the Judge trying to drag the corpse out to dump in the sea. He says he has no idea who it is. The Judge's daughter, Lise Forlacroix, suffers from a mental disorder, and he keeps her locked in her room, next to the fruit loft in which the body was found. Her brother, Albert Forlacroix, had visited the house that night, while the Judge was playing cards downstairs. He suggests that Maigret investigate Marcel Airaud, said to be Lise's lover, but Maigret learns that the waitress at the hotel, Thérèse has a child by Marcel. Marcel disappears, and Maigret learns that the dead man was Émile Janin, a doctor from Nantes.
Judge Forlacroix eventually confesses to Maigret that years earlier, when he'd lived in Versailles, he'd killed his wife's lover and buried the body in a well in his garden. Maigret goes to the house in Versailles, finds the body, and arranges to have Forlacroix's wife, Valentine Forlacroix, come from Nice. With the help of Adine, he locates Marcel Airaud, at Albert Forlacroix's cabin, and has them both brought in. He solves the mystery, that Marcel had planned to marry Lise, three months pregnant, but had called in an old acquaintance, Dr. Janin, to verify that she was mentally fit. When the Dr. had told Albert that she wasn't, she killed him in a rage, then told Marcel she had done it. To protect her, Marcel fled, making it look like he was guilty.
This report prepared by Dana Samson