Pierre is a young lad who often drives with his motorcycle through town and lives in a giant, luxurious mansion with his mother and girlfriend Lucie. One day he discovers a girl spying on him and confronts her: she tells him her name is Isabelle and that she is his sister who lived in a different country abandoned by her father. Angered, Pierre leaves his whole life behind just to be with the poor Isabelle: they find a shabby apartment in Paris and he gets obsessed with writing a novel about the truth of life. The two of them even have intercourse, and Lucie joins them. Crushed by poverty and the rejection of his novel, Pierre kills his cousin, while Isabelle jumps in front of a car.