Bernard Noblet is a modest bank clerk with a passion for billiards. His fiancée Juliette, a schoolteacher, dreams of one day living the high life, and his best friend Roger, a little-known inventor, is equally despairing. After careful consideration, a solution emerges: a cleverly organized hold-up will enable them to live in luxury for the rest of their lives. Bernard soon makes the acquaintance of Bettina and her mother, Madame Ralton, professionals in the "heist" business.

A series of crimes is being perpetrated around a famous singer. Strangely, during each of the crimes, the artist's voice is heard humming his favorite song. Suspicions lead to a chase through a nocturnal Paris, ending in a disused theater where the assassin has lured his final victim.

Police Commissioner Jules Maigret returns to the small village where he spent his childhood at the request of the Countess of Saint-Fiacre, who has received a disturbing anonymous letter.