In the elevator of two large Parisian buildings, a telegraph operator discovers the body of an elderly woman, Madame Mathieu, the owner of both buildings. Boucheron (Lucien Baroux), the local superintendent, is dispatched to investigate, along with his rival, Inspector Lambert of the "Sûreté" (Jacques Baumer). To solve the enigma, they scour staircases and corridors, visiting every floor of both buildings, courtyard and facade. Their gruff rivalry allows them to complement each other in discovering who committed the crime among a gallery of characters, humble or rich, who all have something to be ashamed of... A rare, fast-paced, picturesque comedy set against the backdrop of a police mystery.