In the spring of Kyoko's first year in high school, Eiko, a girl like a sister to her, returns with her younger brother, Jiro. The return isn't that big of a deal, but Jiro has it in his head that he wants to operate on Kyoko! And, what's even more, the two of them are actually from an organization bent on destroying world peace! As if high school didn't have enough problems already, now she has to deal with these two living with her.