Fifteen-year-old Yoko was born during wartime and became separated from her parents while she was still an infant, growing up in an orphanage ever since. The orphanage had found her a family, and on a night of torrential rain, Yoshimura Seiya, a teacher at the orphanage, brought her to the mansion of the Nanjo family. Yoko had originally intended to just take a look at the house and then return, but due to a taxi breakdown, the two, now drenched to the skin, had no choice but to knock on the door of the mansion.