Fine Manners (1978) -- Jean-Claude Guiguet

Camille, a young, provincial, proletarian man works for Helene Courtray, who is still beautiful and seductive. She's a sophisticated, cultivated and well-to-do woman who has engaged him to care for her reclusive son who has spent the past few years voluntarily locked up in his room. An encounter between two persons and two worlds where the relationship of the young man to this closed, strange and unknown universe rapidly turns tragedy.