It's 1949 and former composer Clément Mathieu (GERARD JUGNOT) has agreed to take a supervisor's job at the Fond De L'Etang school for troubled boys. Middle-aged and with his dreams behind him, Clément has taken the job after the previous supervisor packed his bags following one too many run-ins with the various delinquents.
They include Pierre Morhange (JEAN-BAPTISTE MAUNIER), the boy with the angelic face but devilish temperament and Le Querrec (CYRIL BERNICOT) whose prankster booby trap severely injured the school's elderly custodian. Not all of the kids are bad, such as diminutive orphan Pépinot (MAXENCE PERRIN), but that makes no difference to the school's strict headmaster Rachin (FRANÇOIS BERLEAND) who rules with an "action equals reaction" disciplinary philosophy. He and another adult, Chabert (KAD MERAD), believe in doling out the physical punishment, but it doesn't seem to do any good as the boys keep acting up.
Clément decides to take a different approach. Not only does he not turn them in to Rachin when they misbehave, but he also tries to evoke a change of attitude in them by introducing them to music. While the boys -- including Mondain (GREGORY GATIGNOL), a volatile transferee from a detention center -- initially don't want anything to do with that or him, Clément manages to get most of them involved in his unauthorized chorus.
Yet, when he becomes romantically interested in Pierre's single mother, Violette (MARIE BUNEL), and Rachin learns of the supervisor and boys' extracurricular activities, it's anyone's guess whether Clément will be able to complete his goal of improving the boys and their attitudes toward him and others.