Pierre (Louis Garrel) is a seventeen-year-old who lives with his grandmother, and visits his parents in the summer. His dad (Philippe Duclos), is sickly, prematurely aged, and has never taken much interest in him. His mom, Hélène (Isabelle Huppert), is a prostitute who works the streets afternoons. Dad dies unexpectedly. By way of making up for her past neglect, Hélène decides to help get Pierre sexually initiated. Pierre and Hélène share a female lover, one of Hélène's colleagues. That leads up to an incestuous affair between Pierre and Hélène.
The rest is a combination of porn and disrespectful imitation of Fellini. If orgy scenes with participants of indeterminate gender, as well as the visibly aging body of Ms. Huppert, are of interest, then so is this film. One must see this, to believe that she could be in a film this bad. ("Ma Mère," to be sure, is nothing like Louis Malle's "Murmur of the Heart.")