On his first day, new elementary school teacher Shouji Osamu is suddenly embraced by a girl before she runs off. Confused by the random interaction, he continues to class and discovers that the girl, Chise Tahara, is one of his students. Not only is her behavior bizarre, but Chise also seems to know a lot about Shouji—including which university he went to, his parents' occupations, and other little-known facts. Shouji begins to ponder who she could be, but he comes up empty.

Later that night, Shouji enters his apartment's shared bathroom and accidentally walks in on a naked girl showering. Completely distressed, he profusely apologizes to her. When the girl confronts him in his bedroom, he quickly realizes that she is not a random tenant, but Chise. In a shocking turn of events, Chise reveals that she has traveled 12 years back in time to marry him.

According to her, six years from now, Shouji will impregnate another girl from their class and then marry her. This revelation causes Shouji to have a vision of that scenario occurring, and Chise vows to change the future. But after this incident, Shouji cannot shake himself from seeing his students in a sexual light, and Chise's teasing only exacerbates this issue.