Irido Mizuto and Ayai Yume became lovers in middle school. They flirted with each other, disagreed on small issues, became more often annoyed with each other... and ended up breaking up. And so, in the most unlikely way, both of them meet each other again.
"Isn't it clear that I'm the elder brother?". "Isn't it clear that I'm the elder sister?"
Their ex-lover turned out to be the children of their parent's remarriage partner!? The former couple agreed on a 'sibling rule', where the one is aware of the other as the opposite sex loses. However, along with the memories of the past and living under the same roof, how do they would ignore each other!?