A scout leader enlists a friend of hers who broke off her engagement because of the young man's ruin. Both try to instill in the young boys entrusted to them a sense of duty and honor.

Because they brought her back a lost piece of jewelry, a very rich lady accedes to the request of three old people and transforms a vacant lot into a park for the neighborhood children.

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, the local superintendent, is dispatched to investigate, along with his rival, Inspector Lambert of the "Sûreté". 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.