Gérard Favier is a famous composer who has been married to the charming Sylvia for almost 20 years. Still deeply in love, they walk the streets of Paris and, one day, they hear a sonata that Gérard once wrote, played on a piano in a house. Gérard is surprised to find that the piano player is a 20 year-old girl named Monelle. Impressed by her talent, Gérard decides to adopt Monelle as his protégé, but very soon his interest in her takes a less paternalistic turn. It is through a gossip magazine that Sylvia learns about her husband’s infidelity but she is uncertain how to react. After all, he did write a new song for her, "Lovers are alone in the world..."