In 1999, Pierre Morhange (Jean-Baptiste Maunier), a French conductor performing in the United States, is informed before a concert that his mother has died. After the performance he returns to his home in France for her funeral. An old friend named Pépinot (Didier Flamand) arrives at his door with a diary which belonged to their teacher, Clément Mathieu. They proceed to read it together.
In 1949, fifty years earlier, Clément Mathieu (Gérard Jugnot), a failed musician, arrives at Fond de l’Étang (“Bottom of the Pond”), a French boarding school for troubled boys, to work as a supervisor and teacher. At the gate, he sees a very young boy, Pépinot (Maxence Perrin), waiting for Saturday, when he says his father will pick him up. The viewers later learn that his parents were killed in the Second World War during the Nazi occupation of France, but Pépinot does not know this.
Mathieu discovers the boys being ruthlessly punished by the headmaster Rachin (François Berléand) and attempts to use humour and kindness to win them over. When a booby trap set by one of the boys, Le Querrec, injures the school’s elderly caretaker Maxence (Jean-Paul Bonnaire), Mathieu keeps the culprit’s identity from the headmaster, while encouraging Le Querrec to nurse Maxence during his recovery.
On discovering the boys singing rude songs about him, Mathieu forms a plan: he will teach them to sing and form a choir as a form of discipline. He groups the boys according to their voice types, but one student, Pierre Morhange (Jean-Baptiste Maunier), refuses to sing. Mathieu catches Morhange singing to himself, discovers he has a wonderful singing voice and awards him solo parts on the condition that he behaves.
Morhange’s single mother, Violette (Marie Bunel), arrives at the school. When Mathieu goes to explain that Morhange cannot be visited because he has been locked up as a punishment, he finds himself pitying and being attracted to the boy’s beleaguered mother and instead tells her that Morhange is at the dentist. Meanwhile, a cruel, uncontrollable boy named Mondain (Grégory Gatignol [fr]) arrives and begins causing trouble by bullying the others and generally being rebellious. After stealing a watch, he is locked up for two weeks.
The choir is improving rapidly with Morhange as its lead soloist; the children are happier, and the faculty less strict—even Rachin begins to loosen up, playing football with the boys and making a paper aeroplane. After Mondain is released from lock-up, he runs away. At the same time, all the school’s money disappears. After Mondain is captured, Rachin beats him, and Mondain in turn attempts to strangle Rachin. Rachin hands him over to the police, still not knowing the location of the stolen money, and disbands the choir. This causes Mathieu to teach his choir “underground”, practising at night in their dormitory.
Mathieu continues to meet Morhange’s mother, who is unaware of his attraction to her. He plans to help her son win a scholarship to the music conservatory in Lyon. One day she blithely informs him that she has met someone: an engineer. Mathieu is dejected but expresses his feigned happiness and watches her leave in the engineer’s car.
The Countess, a sponsor of the school, finds out about the choir; they perform before her and others, and Morhange enchants the audience with his solo. Mathieu discovers that another boy, Corbin, stole the money that Mondain was accused of taking. Despite this, Rachin refuses to accept Mondain back at the school.
When Rachin departs to accept an award from the board after taking credit for the choir, Mathieu and Maxence suspend classes and take the boys on an outing. While they are out, Mondain returns and sets fire to the school. Mathieu is fired for breaking the rules, even though he saved the boys’ lives, and Maxence is suspended. As Mathieu leaves, the boys—forbidden to say goodbye—lock themselves in their classroom, sing and throw farewell messages out of the window on paper planes. Touched, Mathieu walks away, musing about how he has failed and nobody knows of his existence.
Back in the present, the adult Morhange finishes reading the diary and recounts what happened afterwards: he won his scholarship to the conservatory, and the headmaster Rachin was fired for his brutal discipline. Mathieu, Pépinot relates, continued to give music lessons quietly for the rest of his life.
The final scene (in the past again) shows Mathieu waiting for his bus after being fired. As he boards it, he looks back and finds Pépinot running after him, insisting that he come too. Initially, Mathieu refuses because it is not allowed, and he leaves Pépinot behind. Suddenly, the bus stops and Mathieu gives in: the two board the bus together. Pépinot finally got his wish, for he and Mathieu left on a Saturday, and Mathieu raised him.