At a certain junior high school, a certain boy and girl became a couple. They did things that most couples their age do, like make out and disagree about trivial things. But their days that were once filled with excitement and joy had faded away, and they mostly just got annoyed at each other. Therefore, they decided to use graduation as a chance to end things, giving them both a clean slate for high school. Then, right before their high school lives are about to begin, the two of them, Mizuto Irido and Yume Ayai, meet each other again in the most unexpected way…Mizuto’s father remarries, and his stepmother’s daughter is his ex-girlfriend!

“Isn’t it obvious that I’m the older brother?”
“Isn’t it obvious that I’m the older sister?”

Wanting to put their parents' feelings first, Yume and Mizuto make a “sibling rule” that whoever gets aroused by the other first loses. But with seeing each other after just getting out of the bath, going to school and walking home together, along with memories of their past and living under the same roof, how exactly are they supposed to manage that?!