From Artificial Eye website:
Claude Chabrol’s taut thriller stars Isabelle Huppert as the villainous spider at the centre of an intricate and murderous web of deception. Huppert plays Mika, wife of celebrated pianist André Polonski (Jacques Dutronc) and stepmother to his son, Guillaume, whose mother died in a car wreck on his sixth birthday.
Their lives are interrupted by the un-expected arrival of Jeanne (Anna Mouglalis), a young woman who has learned that she was almost switched at birth with Guillaume whilst in hospital. Also a pianist, Jeanne harbours a suspicion that she may be André’s daughter. André undertakes to continue her piano tuition, but, on entering the Polonski family, Jeanne begins to notice the icily controlled Mika behaving strangely. Her suspicions aroused, Jeanne begins the dangerous task of unravelling Mika’s dark past of secrets and lies.