Shin'ya Fujimi navigates a tedious school routine until Akane—once arranged to marry his older brother—joins his class. Childhood memories resurface, including their promise to reunite as adults. Fueled by lingering feelings and this vow, he resolves to pursue her. He solicits intimate guidance from his sister-in-law, Sayumi Fujimi, the school nurse. Applying Sayumi's techniques with Akane, their escalating physical intimacy dissolves emotional barriers between them. This evolution reverberates through their social circle, shifting other relationships in turn.