 When the young Jeanne finds out that she could have mistakenly been exchanged, from her cradle, with another newborn (the son of a famous piano player), she decides to contact this family, not really thinking that this could be the truth. After she tells this to the presumed father and the rest of the family, even the son of the piano player starts having more than one doubt about his origins, and all this will lead to the discovery of a murder committed many years before by the second wife of the musician. But all the story is just a pretext to put in evidence the misdeeds and the hypocrisy of the French upper class members (okay, in the film they are from Switzerland... but it is essentially the same). Chabrol belongs to this class, that s why he knows the subject so well, so the blemishes are noticed and shown to the audience without pity; things like the coldness between the family members - hidden by a false happiness - because this is what the good society wants, the lack of real feelings, the unbearable  bon ton  of the mother, and so on.