Plot
In Matsumoto, Nagano, Naho Takamiya, a second-year high school student, receives letters sent from herself 10 years into the future. Her future self asks her to prevent her "biggest regrets", which has something to do with the new transfer student from Tokyo, a boy named Kakeru Naruse. At first skeptical, Naho begins to believe the letters as they accurately predict events. When 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 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 to him pursuing a relationship with an upperclassman, and insisting they watch the fireworks alone. Naho learns from the letters in the future Kakeru is dead, having committed suicide one day after Valentine's Day in the same school year in which he enrolled.
During the Bon Festival, Kakeru explains to Naho the reason for his absence: his mother died by suicide because of a mental breakdown caused by him leaving her for friends, and not taking her to the hospital on the day of enrollment as he promised. This causes Naho to also feel guilty, as she feels they share responsibility for Kakeru's mother's death. Later on, Suwa tells Naho he has also received letters from his future self, asking him to be a good friend to Kakeru and save him from his death. Suwa hides the fact he and Naho are married and have a child in the future, as he realizes Naho and Kakeru are in love. Although he has feelings for Naho, his future self realizes by saving Kakeru, he is giving up a future with Naho. Because he knows of their feelings, he listens to the letters and supports them as much as possible. Later, Azusa, and Hagita, and Chino admitted they have received similar letters. They all agree though they may not be able to change the fate of their future selves, they may create a parallel universe where Kakeru is still alive.
To cheer up Kakeru, the five friends organize a celebration for his seventeenth birthday, in which Kakeru confesses his feelings to a surprised Naho. The five also join the relay race team to back Kakeru. With his friends' encouragement, Kakeru wins the race. As his reward for winning the race, Kakeru kisses Naho. However, on New Year's Eve, Naho and Kakeru argue over the latter's grandmother's health. The two become distant over the following weeks, up until the day Kakeru is supposed to die by suicide, 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. The five search throughout Matsumoto and manage to stop him from getting hit by a truck. Kakeru apologizes, telling them he had been thinking of suicide, but at the last second decided not to after realizing it would mean he would never see his friends again.
