Billy mistakes another apartment for his own on the eve of his wedding and wanders in to meet a woman who immediately mesmerizes him, causing all sorts of problems in what used to be an uncomplicated life. After Billy (Richard Berry) and his bride-to-be Nina (Zoe Chauveau) move into a housing project still in its finishing stages, Billy walks into the wrong apartment when he forgets which floor he is supposed to be on. When he sees Viviane (Brigitte Fossey) he is transfixed by her, and since her husband has just ended their marriage, she is particularly susceptible. Without explaining where he actually lives or that he is getting married, Billy seduces Viviane, but then does not have the courage to break off his wedding. When Viviane rejects their affair and takes off for Paris, Billy goes in search of her -- not knowing that Nina's father has caught on to Billy's extra-marital activities and would like to express a few viewpoints of his own in that regard.