Le Bon Dieu sans confession ( Good Lord Without Co (1953)

Stuck in a loveless marriage, bourgeois industrialist Francois falls in love with Janine, another man's wife. Francois sets up Janine as his mistress, and she, mercenary soul that she is, likes the set-up so much that she continues the relationship even when her own husband returns from WW II. In the long run, however, Janine is the loser in the situation.