The older brother, Ryuta, is a semi-recluse who has been having a sexual relationship with his younger sister, Nenne, a female student. Nenne knows it's wrong, but she can't refuse Ryuta, who is so kind to her, and although they haven't had intercourse for years, they continue to have oral sex. One day, Kameda, a friend of Ryuta's from his school days, comes to visit. Kameda has his eye on Nenne's big breasts, and he plans to threaten her by using her relationship with Ryuta as a threat and pin her down...