A certain boy and girl in middle school became lovers, flirted with each other, disagreed on trivial things, became more frequently irritated with each other rather than excited... and ended up breaking up at graduation. And so the two of them, Mizuto Irido and Yume Ayai, wound up meeting each other in the most unexpected fashion. "Isn't it obvious that I'm the elder brother?" "Isn't it obvious that I'm the elder sister?" The children of the partner of their parents' remarriage were ex-lovers?! The former couple, mindful of their parent's feelings, decided on a "sibling rule" where the one who becomes conscious of the other as the opposite sex loses, but... When they encounter each other in the bathroom, commute to and from school together... along with the memories of the past and living under the same roof, just how do they disregard each other?!

Kou Yamori seems like a typical middle school student on the surface. Relatively good at studies and amiable with his classmates, he puts a lot of effort into maintaining this facade. One day, however, he decides to stop pretending and quits school, developing insomnia as a result of having no daytime outlet for his energy. When taking walks alone at night, he feels marginally better, though he is aware that his inability to sleep should be considered a serious problem. On one such walk, Kou meets a weird girl, Nazuna Nanakusa, who diagnoses the cause of his sleeplessness: despite making changes in his life, he is still holding himself back from experiencing true freedom. She says that he won't be able to sleep unless he is satisfied with how he spends his waking hours. When it appears that she has resolved his current worries, Nazuna invites him back to her apartment to share her futon. After a while, unaware that he is only feigning unconsciousness, she leans over him—and bites his neck!