Eriko, Chisato's younger sister, comes to visit her older sister Chisato's house. The four of them are excited to meet again after a long time, but by chance, Eriko's son Tatsuya witnesses Chisato masturbating. Chisato attacks her nephew as her desires take over, and things start to go haywire, leading to a massive incestuous relationship between the two sets of parents and children.