François Hainaut, a fifty year old philosophy teacher, is content with his work and his marriage. But then Mathilde Tessier, a 17 year old student, crashes into his life. The teacher becomes concerned by the girl's troubled background and realises that she may be expelled from college if she doesn't change her attitude. So he takes it upon himself to coach the rebellious teenager. Very quickly, the student-teacher relationship turns to carnal passion, and then obsession. Knowing that the affair cannot last, François tries to put an end to it. Mathilde's reaction proves to be devastating...