Matthieu lives in the countryside beside a river and spends the summer alone with his parents. Fifteen years old, he dreams about adventure and about meetings. Little by little, he becomes foreign to life of the household: he deceives his boredom by defying prohibitions and by taking refuge in a world where he confronts his childrhood games and his desire to grow-up. His heart begins to beat when the reality catches him uo or when his imagination plays tricks on him.