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..."