In The Shadow Of Women

Pierre and Manon are poor. They make documentaries on a shoestring budget and put food on the table by doing odd jobs. Pierre meets a young intern, Elizabeth, and she becomes his mistress. But Pierre will not leave Manon for Elisabeth - he wants to keep both. One day, Elisabeth discovers that Manon has a lover. She tells Pierre... Pierre goes back to Manon, since it's her that he loves. And as he feels betrayed, he pleads with Manon and forsakes Elizabeth.

Manon immediately breaks it off with her lover. Presumably it's because she loves Pierre.