A young woman inherits her family's historic home on the coast of Brittany, a house she has not been to since her father died there when she was age 12.

The children of a small European mountain village end up being its only residents, soon after the adults cannot take their behavior any longer.

Esteban and Maxime are inseparable. Their families have always lived in the same Parisian building and Maxime, who has lived alone with his mother, a hard-working attorney, since his father died, has always felt part of Esteban's happy, friendly family. The boys have known each other since birth and consider themselves more than friends, they are brothers. The two friends have secretly made a small attic room the center of their world, where they invent characters and store their disguises and most treasured possessions. But their friendship is threatened when Maxime learns to his dismay that Esteban and his Spanish-born family are moving back to Spain. The two boys dream up a plan that they must soon put into action.

Set in 1992, a manager Vincent has to run a children's holiday camp for three weeks and to face the unexpected concerning the place, his colleagues, various problems linked to children about the rooms, trips, their belongings...

Several boys aged seven to fourteen, have formed their own secret support group as a result of parental neglect and abuse. The group is however more keen on revenge against the adults on their island. They break into houses and steal items, not for money, more for the thrill. They vandalize homes and torment the occupants. One man decides to seize the opportunity to do away with his wife and blame it on the intruders.

Based on the 2000 book of the same name by Emmanuel Carrère, it is inspired by the real-life story of Jean-Claude Romand. L'Adversaire's protagonist Jean-Marc Faure (Auteuil) pursues an imaginary career as a doctor of medicine in a plot more closely based on Romand's life and Carrère's book than was Laurent Cantet's 2001 film L'Emploi du Temps. The film was nominated for a Palme d'Or at the 2002 Cannes Film Festival.