Marc Lafarge, 13, is idolized by his brother Thomas, 20, future veterinarian and amateur boxer. Thomas has a soul of justice. Thus, returning home with Marc after a fight, he hits a little thug who has just robbed an old lady.

Two young men, Zim and Jean-Pierre, both sentenced for a minor offence, recognize one another when they are released from prison on the same day. One wants to reform, but his friend wants to stay a crook. The first one gets a job and begins improving his life, while the young criminal ends up incarcerated for stealing cars.

Antoine is a violinist in love with sweet Sophie. Problem: his sweetheart is the daughter of the master of the municipal harmony, a stubborn man who swears only by the brass. Never mind: Antoine decides to ask him for his daughter's hand and to win his place at the orpheon. Will the two men be able to get in tune?