Melanie gets her revenge, she destroys both the career and the marriage of Ariane, and the musical ambitions of her son.
As a child, Melanie (Déborah François) had her musical ambitions ruined by Ariane (Catherine Frot), who as a jury president for conservatory admissions signed an autogram instead of listening to the piano play of Melanie, who then got distracted and messed up, and after that never played piano again.
As an adult, Melanie manages to work in the office, and later in the house of Ariane's husband, who is a rich and successful attorney who has married concert pianist Ariane as a status symbol. Melanie is asked to baby-sit their son, and due to her abilities to read music notes, she becomes the page turner of Ariane. Ariane has a "stage fright" problem since being the victim of a hit-and-run car accident. Thanks to Melanie, Ariane regains confidence, but becomes dependent of Melanie, and is also somehow physically attracted to her.
Melanie "helps" the son with his piano exercises and sets up the metronome to run faster, so that his arm hurts, and that he doesn't really enjoy playing anymore.
Melanie leaves Ariane alone at an important concert, so she plays poorly and her agent drops her. Ariane forgives her after an apology. At the last day of her stay, Melanie asks Ariane for an autogram. Ariane writes one in which she confesses her love for Melanie.
Melanie leaves early and drops the autogram on the desk of Ariane's husband. Her husband finds the note and shows it to Ariane, who faints. In the last scene, Melanie is shown walking away with a smile on her face.