Maîtresse (1976)

A common thief (Depardieu) breaks into the house of a professional dominatrix (Ogier), and begins to help her "train" her clients. Though this world is alien to his experience, he finds himself falling in love with her. Eventually he discovers that she does this in order to support her son, and he attempts to help her out of this life, which she is not sure she really wants to leave.