High schooler Miki Koishikawa is blindsided when her parents announce a divorce-and-remarry swap with another couple, merging her household overnight with Yuu Matsuura, the son of her mother's new husband. Forced to share a roof and a school with her new stepbrother, Miki starts falling for him even as her tennis-team senior Ginta Suou and Yuu's own tangled history complicate things. Jealousy, old relationships, and a family still learning how to be one keep the two circling what they actually are to each other.