17-year-old Taisei Mikami wanders the streets in search of a woman, ignoring his classmates who are studying for their entrance exams. Taisei is attracted to his 14-year-old younger sister, Nao, who has recently become more beautiful. In fact, just before her father died, Nao was told by her best friend that she was her daughter and that she was not a real brother and sister to Taisei.