Uwakizuma: Inran dônôkai (1996)

Misako is invited to a party celebrating the wedding anniversary of her former teacher at school, Professor Kitamura, and his wife, Hanae. Her husband, Akira, waits for Misako to return home, drinking beer by himself, but she never comes back. Her younger sister, Kana, lives on the second floor, and it seems that her boyfriend, Toru, has come over today and is having sex with her. The next morning, Misako finally returns home. Akira is furious, but she can't forget how much fun she had at the party. What did she see and experience there?