In Matsumoto, Naho Takamiya, a second-grade high school student, receives letters addressed from her own self 10 years into the future. She is asked to prevent her “biggest regret”, which has something to do with the new transfer student from Tokyo, a boy named Kakeru Naruse. While skeptical, Naho begins to believe the letters as they predict some events that she would in no way be able to experience. While the letter asks her not to invite Kakeru to go out for the first day, Naho and her friends: Hiroto Suwa, Takako Chino, Saku Hagita, and Azusa Murasaka, decide to invite him anyway. Kakeru ends up not attending school for the next two weeks.
To prevent another such mistake, Naho resolves to do what the letters order her to do, such as volunteering on a softball round, encouraging Kakeru to join the soccer team, objecting him into pursuing a relationship with an upperclassman, and insisting that the two watch the fireworks alone. Naho also reads the letters further and learns that in the future, Kakeru is dead of because of suicide, which happened around Valentine’s Day in the same school year that he enrolled in.
During Bon Festival, after insisting him to open up, Naho is told by a tearful Kakeru the reason for his two-week absence: his mother committed suicide because of a mental breakdown caused by him calling her annoying for asking him on the day of enrollment, when he was with Naho and the others. This sends Naho into a deep guilt, as in a way, she and her friends have a responsibility for Kakeru’s mother’s death. A few days after, Naho is met by Suwa, who assures her that he also received the same letters from his future self, who asks him to do the same thing as Naho’s, although he purposefully hides the fact that he and Naho are married and have a child in the future as he wants Naho to become closer with Kakeru, whom she is in love with. Later, it is revealed that Azusa, Hagita, and Chino also received the letters. They all agree that though they ma