Plot
Detective Chief Inspector Maigret of the French Judicial Police has been transferred to Luçon in the Vendée region. He travels to the coastal village of L'Augillon to investigate reports of a corpse seen in the home of Judge Forlacroix, a retired justice of the peace from Versailles. That night he sees Forlacroix trying to dispose of a body.
Forlacroix explains that he did not kill the man: he found the body in his home and has no idea who he was. He had been playing cards with friends in the library downstairs. When he went upstairs to go to bed he found his son, Albert, sitting at the top of the stairs. They had a brief conversation before Albert went home. The following morning Forlacroix found a body in a room upstairs he calls "the fruitery". The fruitery adjoins the bedroom of his daughter, Lise, who he locks in her room at night because she is a sleepwalker. He decided to throw the body into the sea to avoid complications.
Maigret finds fresh muddy footprints in Lise's room, which Albert says are those of Marcel Airaud, a fellow mussel farmer. Marcel admits that he was at Forlacroix's home the previous night but not on the night of the murder.
Forlacroix tells Maigret that Lise has bouts of madness, and he is afraid people will let their imaginations run wild. Marcel flees to the marshes around Marans, and Maigret orders the police to search for him. A local woman tells Maigret that Marcel is in love with Lise but does not take advantage of her the way other men do. She explains that men can easily get into Lise's room by climbing the terrace and entering through the window of the fruitery.
The dead man is identified as Dr Janin, a psychiatrist from Nantes. Forlacroix denies knowing Janin but confesses that 15 years earlier he killed his own's wife's lover and cemented the body into a disused well on his property in Versailles. Maigret and Forlacroix take Lise to a nursing home. Maigret then arrests Forlacroix and goes to the property in Versailles where he finds a skeleton in the well.
Maigret learns that Marcel had known Janin in the navy. He tells the examining magistate that Forlacroix consulted doctors about his daughter's condition and was told that it was likely to get worse and it would be wise to marry her off. Forlacroix and Marcel agreed Marcel would marry Lise in two month's time. Maigret speculates that Marcel wanted to be certain about Lise's mental health, so he consulted Janin.
Maigret learns that Albert has given Marcel shelter on his property, so he puts Marcel under surveillance. Albert tells Maigret he has long been estranged from Forlacroix and knew from the age of 16 that he was not his biological father. Marcel was once his friend, but they had a falling out when he discovered Marcel was having sex with his sister.
Maigret suggests to Albert that he and Marcel arranged for Janin to secretly examine Lise, sneaking upstairs while Forlacroix was playing cards in the library. Albert admits that he has keys to Forlacroix's front door and Lise's room but says he was only at Forlacroix's home for five to ten minutes on the night of the murder.
Maigret discovers that Lise is pregnant and that Marcel had been seen waiting outside Forlacroix's house just before midnight on the night of the murder. He has Marcel arrested and questions him, telling him he knows that Albert went to Lise's room with Janin, leaving Marcel waiting outside Forlacroix's home. He suggests the doctor examined Lise then told Albert in the fruitery that Lise was pregnant and incurably insane.
Maigret calls Albert back in to confront Marcel. Albert denies killing Janin, saying it was Lise. But Maigret accuses Albert of killing Janin in a rage and suggests he is mad like his sister. Albert charges at Maigret, but Marcel wrestles him to the ground. The police handcuff Albert. Marcel confirms that Albert had told him that Lise had killed Janin, so he had fled to divert suspicion from her.
