The story is about a boy named Kouta who attends St. Narihari Academy, a school for students from kindergarten to university with his beloved older sister, Suzuka. However, Suzuka has been secretly harboring sexual desires for her brother, and after confirming his interest, she takes his virginity. When Suzuka's classmate, Himari, finds out about their relationship, she blackmails Suzuka into letting her have sex with Kouta too.