Yuuta Yamauchi is an ordinary high school student studying for exams throughout his summer break. His older cousin, Kei Higashide, feels sorry for him as her "little brother" and dedicates her time and patience to tutoring this relative in need. One day, Yuuta manages to photograph his classmate and crush, Youko Kaji, when a gust of wind lifts her skirt. Upon discovering this, Kei laughs at Yuuta's juvenile infatuation and begins teasing him about his sexual preferences. Moving from words to actions, she offers him her own mature body—and Yuuta quickly takes advantage of the opportunity. As summer progresses and the relatives spend more time together, their initially dull tutoring sessions soon evolve into a passionate and fiery relationship.