Mizuho Suga believes that people can be categorized into particular groups, behaving true to form. Since she can predict how others will act, she feels that she understands them. While attending the funeral of a former classmate, Haruka Origichi, she finds it unfortunate that someone died at such a young age. But her impression of Origichi had been that she kept to herself and was unnoticeable, and as such, her life must also have been unremarkable.

After the funeral, Suga is approached by Origuchi's mother, who mistakenly assumes that the two girls were best friends. The elder Origuchi wants to know about an ex-boyfriend that her daughter stubbornly tried to keep secret only years ago; apparently, Origuchi aborted their child before she became ill. Suga agrees to investigate this mystery, gradually gaining the assistance of former upperclassman Takashi Yanai, class celebrity Madoka Setouchi, faithful tag-along Remi Nishida, and enigmatic playboy Hikaru Narumi.

Although Suga was not close with Origuchi when she was alive, she is strangely determined to know the other girl better after her death. She starts to piece together the events that happened in their second year of high school. As Suga seeks the truth from those suspected to be involved with Origuchi, she must learn how to open up to others and be more honest herself.