Erika Kohut (Isabelle Huppert) is a piano professor at a Vienna music conservatory. Although already in her forties, she still lives in an apartment with her domineering mother (Annie Girardot); her father is a long-standing resident in a lunatic asylum. Throughout the film, the audience is gradually shown truths about Erika's private life. Behind her assured façade, she is a woman whose sexual repression verges into full-fledged desperation and is manifested in a long list of paraphilias, including (but by no means limited to) voyeurism and sadomasochistic fetishes such as sexual self-mutilation.
Upon meeting Walter Klemmer (Benoît Magimel), a charming 17-year-old engineering student from a bourgeois background, they begin to obsess one another. Even though she initially attempts to prevent consistent contact and even tries to undermine his application to the conservatory, he eventually becomes her pupil. Like her, he appreciates and is a gifted (if distinct) interpreter of Schumann and Schubert.
She destroys the musical prospects of an insecure but talented girl, Anna Schober, driven by her jealousy of the girl's contact with Walter -- and also, perhaps, by her fears that Anna's life will mirror her own. She does so by hiding shards of glass inside one of her coat pockets, permanently damaging Anna's hands and ruining her aspirations to play at the forthcoming jubilee concert. Erika is then wholly sympathetic when the girl's mother (Susanne Lothar) asks for advice on her daughter's recuperation. (The sub-plot of the pupil and her mother, mirroring the main relationship in the film, is absent in Jelinek's novel.) In what clearly qualifies, at that point of the film, as dramatic irony, the girl's mother rhetorically asks Erika who could do something so evil.
Walter is increasingly insistent in his desire to start a relationship with her. But when she finally acquiesces, Walter is unwilling to indulge her violent fantasies, which repulse him. The film climaxes, however, when he finally does. He attacks her in her apartment in the fashion she let him know she desired, violently raping her as she lies prostrate after receiving a beating from him. She discovers, finally, that the reality of her desires does not match her conception of them.
Shortly thereafter, minutes before the concert in which she is supposed to fill in for Anna, Erika tries to enact some speculable form of revenge or call of attention on Walter, and when confronted with his hypocritical, smug indifference she puts a knife into herself and promptly leaves the auditorium. Her onscreen injury is not severe, but the implications of the whole ending scene leave it clear to the audience that more serious self-inflicted harm is a matter of course.