Naho Takamiya is a sixteen-year-old high school student in the city of Matsumoto whose life is turned upside down when she receives a letter one morning, allegedly from her twenty-seven-year-old self. Initially dismissing it as a prank, she becomes a believer when it correctly predicts that a new student, Kakeru Naruse, will transfer into her class. The letter from the future is a desperate plea filled with regret. It explains that in the original timeline, Naho and her friends failed to see the deep depression Kakeru was hiding, leading to his death by suicide shortly after the school year ended. The letter provides specific instructions on actions to take and mistakes to avoid, all with the goal of saving his life.

As the story unfolds, Naho enlists the help of her close-knit group of friends: the kind and athletic Hiroto Suwa, the outgoing Takako Chino, the quirky Saku Hagita, and the cheerful Azusa Murasaka. Unbeknownst to Naho at first, each of them has also received similar letters from their own future selves, compelling them to work together to change fate. The narrative follows their daily high school life, navigating the complexities of class festivals, soccer games, and study sessions. However, these ordinary events are charged with the heavy knowledge of what is to come. The group faces several key moments detailed in the letters, such as ensuring Kakeru joins the soccer team to give him a sense of belonging, preventing him from dating an upperclassman who would ultimately hurt him, and successfully celebrating his birthday—an event the future versions of themselves deeply regretted overlooking.

A central emotional conflict involves Naho’s growing romantic feelings for Kakeru, which are mirrored by Suwa’s long-standing love for her. The letters reveal that in the future, Kakeru died believing no one needed him, and that Naho was left with the unbearable weight of words left unsaid. To counter this, the friends make a concerted effort to include Kakeru and show him they care, even when he tries to push them away due to his trauma surrounding his mother’s suicide. The live-action adaptation follows their struggle as small victories begin to alter the course of the original timeline, leading to a tense climax where the friends must intervene directly to prevent Kakeru from isolating himself completely on the day he originally took his life.