Kôshoku engi: Nurenure (1976)

A middle-aged couple is feeling bored with their sex life. As a solution, they hire a live-in helper for a young couple, hoping to spy on their sex life to get some excitement. Meanwhile, the young couple's husband has become impotent due to psychological reasons, and the young wife is completely frustrated. They applied hoping that spying on the middle-aged couple's intense sex life might stimulate her husband's impotence. Thus begins the strange cohabitation of the two couples with their own motives intertwined.