The Temptation of Isabelle (1985)

Bruno devises a mean to test the love of his girlfriend Isabelle. Since Isabelle had been in love with Alain in the past, Bruno invites Alain to a surprise birthday party for her at a hotel -- what better way to judge her feelings than to get them together? Alain arrives with his current girlfriend Lio. Soon after the two couples start the evening off, their polite exteriors deteriorate.