Ritsu Aida (17), a second-year high school student, begins living alone at his grandmother's house after his parents are transferred overseas for work. However, on his very first morning there, he wakes up to find an unfamiliar young man, Nagi Asakura, standing beside his bed. Calmly introducing himself with, ""Because I'm a ghost,"" Nagi seems to be visible only to Ritsu. And so, their strange life together begins. Often isolated at school, Ritsu gradually grows closer to Nagi, who stays by his side morning and night, sometimes even causing little commotions just to get Ritsu's attention. As Nagi's past and lingering regrets slowly come to light, Ritsu finds his heart beginning to race. Then, on a snowy night, Nagi suddenly disappears. Left behind alone, Ritsu finally comes to realize the true nature of his feelings — and where they may lead.