At a family wedding, two distant in-laws—Marthe (Marie-Christine Barrault) and Ludovic (Victor Lanoux)—meet and immediately hit it off. While waiting around during the reception, they discover that their unfaithful spouses, Pascal (Guy Marchand) and Karine (Marie-France Pisier), have sneaked off together to have a secret sexual encounter. What begins as a platonic, shared bond between Marthe and Ludovic deepens over subsequent family events, growing into an open, passionate extramarital love affair that scandalizes their hypocritical relatives.