A certain boy and girl in middle school became a couple, flirted with each other, disagreed on trivial things, became more frequently irritated with each other… and eventually broke up upon graduation. Now in highschool, the two of them, Irido Mizuto and Ayai Yume, wound up meeting each other in the most unexpected fashion. The former couple, brought together through parental remarriage, decided to be mindful of their parents' feelings and came up with a 'sibling rule'—where the first one to become conscious of the other as a member of the other s*x 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!?