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 may not be able to change the fate of their sender selves, they may create a parallel universe where Kakeru can end up living.
To make Kakeru happy again, the five friends organize a celebration for his 17th birthday, in which Kakeru confesses his feelings for Naho, much to her surprise. The five also join up the relay race team to back Kakeru. With his friends’ encouragement, Kakeru ends up winning the race. As his reward for winning the race, Kakeru kisses Naho. However, during New Year’s Eve, Naho and Kakeru get into a conflict over the latter’s grandmother’s health. The two become distant over the following weeks, up until the day that Kakeru would commit suicide according to the letters, as Naho manages to confess her feelings as well as telling Kakeru to share his plan for suicide.
On the night of Kakeru’s supposed death, the friends’ plan to meet up is interrupted as Kakeru does not arrive on time. In panic, the five search through Matsumoto and find him almost getting hit by a truck. Kakeru then apologizes, telling them that he almost thought of suicide, but at the last second decided not to after realizing that doing so would mean that he cannot meet with his friends again.