Juste avant la nuit (1971)

Juste avant la nu (1971), also known as Just Before Nightfall, tells the story of an ad exec who's having lots of sex with Anna Douking. Schwing! There's one small problem with bonking Ms. Douking, however: she's the wife of the exec's best friend! Nevertheless, Michel Bouquet and his mistress keep going with the illicit sex...until one of the couple's S&M games goes too far and Mike strangles poor Anna. He tries to get away with it, and it seems like he might escape without doing time. But will his concience get the best of him?