A boy and a girl started dating in middle school. They were lovey-dovey at first, but due to a misunderstanding over something trivial, they started feeling more frustration than excitement… In the end, they broke up when they graduated. However, Mizuto Irido and Yume Ayai meet again in a surprising way before they start high school.

“Obviously I’m the older brother here.” “Obviously I’m the older sister here.” The child of their parent’s new spouse is the ex they just broke up with?! Out of consideration for their parents, this former couple comes up with a “sibling rule,” which is: “Whoever starts seeing the other as potentially attractive loses.” But they sometimes run into each other right after a bath. They’re also alone on their way to and from school, so…

[Source: Crunchyroll]