Mei-chan, an innocent high school girl, gets a new stepdad. One day, her mother leaves for a three-day trip, leaving them alone together. Although Mei has always relied on her stepdad for help with her studies, he suddenly begins acting strangely affectionate, as if he's been waiting for this opportunity. When he discovers that Mei has a crush on a classmate, he starts plotting something behind her back. Is it jealousy, or has everything been part of his plan all along? Using "sex education" as an excuse, he gradually seduces Mei, leading their relationship into uncharted territory.