It's obvious that the penniless Heurtin murdered the old woman for her money - isn't it? Maigret doesn't think so, and deliberately allows him to escape from custody to learn the truth.
Evidence points to the brother of one of Maigret's officers as the killer of his mother in law. But the suspect's son is missing. At first the boy of ten is thought to be covering up evidence of his father's guilt, but the boy is leading a clever trail to help the police identify and capture crook.
A serial killer is at work in a working-class district, his victims all women. Maigret soon identifies the killer, but what intrigues him is the motive.
Maigret is working round the clock on a baffling spate of bank robberies. When an intriguing murder occurs, Maigret is ordered to concentrate on the robberies but, being Maigret, can't help taking an interest in the murder.