Akiko Yasutomi, an 18-year-old high school student, lives in poverty with her mother Hisako, but she has feelings for her cousin, a medical student named Masato Miki, who lives upstairs. However, when Masato becomes a part-time tutor for Akiko's classmate, Natsuko Shigeno, and Akiko becomes close to Natsuko, Akiko feels lonely. These feelings also help her to lose her purity to Junichi Yoda, whom she met by chance through an accidental mistake.