In a smoky Montparnasse cafe, Willy Ferrièrre, Gaston Jacquet, a gambler living beyond his means, openly muses that he would pay 10,000 francs to hire someone to kill his wealthy aunt so he can claim the inheritance. An unseen correspondent slides him a note accepting the offer, setting in motion a thread that pulls the detective world into a tightening snare. Later that night, a low-life criminal, Joseph Heurtin, Alexandre Rignault, infiltrates the aunt’s house, following a trail of footprints in the dark and a map toward the aunt’s bedroom, hoping to discover a hidden stash of money. In the dim glow, he stumbles upon the murdered woman and, with blood on his hands, leaves his fingerprints behind. A shadowy figure emerges—Radek, Valéry Inkijinoff—a Czech émigré who promises to clean up the prints and tells Heurtin to go into hiding. After Heurtin departs, Radek exits the house and locks the door, choosing not to wipe away the crime scene.
The case lands on the desk of Inspector Maigret, Harry Baur, whose crew follows the fragile clues to Heurtin’s parents’ Versailles home, where the fugitive has been sheltering. Under arrest, Heurtin protests his innocence and insists there was another man present in the murdered woman’s house. The magistrate assigned to the case remains wary, ready to prosecute, but Maigret sees through the surface and lets Heurtin escape, dispatching a tail to keep him in sight. Heurtin’s route loops back to the Montparnasse cafe, where Radek, now blackmailing Ferrièrre, draws Maigret’s attention. Maigret suspects a third party’s involvement and engages in a tense sequence of encounters with Radek, who all but admits to the “perfect crime”, though Heurtin continues to elude full surveillance.
Though Maigret is formally removed from the case for allowing Heurtin to slip away, his instinct tells him Radek will lead him to the original mastermind. A return to the cafe follows, with Maigret sharing a table with Ferrièrre and his mistress, Edna Reichberg, Gina Manès. Radek—dying from tuberculosis—cones into a confrontation with Ferrièrre and Edna, pulling Edna away to his apartment while Ferrièrre remains largely a bystander. In the apartment, Radek bares his loneliness, his passion for Edna, and his bitter contempt for social pretenders like Ferrièrre, revealing a personal motive that further unsettles the case. As Maigret and his team close in, Heurtin again crosses paths with Radek, and a violent clash erupts. Radek shoots one of Maigret’s men, and in the fatal confrontation that follows, both Radek and Heurtin are killed. Maigret, calm and resolute, leaves the building with Ferrièrre under arrest, his investigation finally edging toward a form of closure for the tangled web of deceit.
