Garçonnières très spéciales (1981)

Francoise's husband Jean isn't very interested in her. But her friend, Marie, and her husband, Jean-Claude, enjoy using an apartment with a two-way mirror into the bedroom of the adjoining apartment. There they can have sex and watch other couples and threesomes have sex at the same time. Marie sympathizes with her friend's plight and gives her the key to the apartment. Françoise is reluctant to make full use of the 'facilities' at first, but eventually succumbs to the temptation of using it with an insurance man Bruno, a female neighbor Valerie and Vincent, a long-time frequenter of the apartments (Richard Lemieuvre). While she is with the last, she sees her husband going at it with next door woman. She ends up having a threesome with Vincent and Mathias while her husband has two girls on the other side of the mirror. She has much better sex with her husband as a result.