The story begins with Asaka Natsumi, a new transfer student, introducing herself on her first day of high school. To her bewilderment, her entire 1-C class and homeroom teacher, Shiratori-sensei, erupt in cheers and tears. Class representative Andou later explains the bizarre situation: Natsumi is the only one experiencing this year for the first time. Everyone else, including Shiratori-sensei, has been trapped in a 12-year time loop, repeating the same high school freshman year.
Natsumi, initially skeptical, is convinced after a week when Andou's notebook, detailing a year's worth of hourly weather forecasts, proves perfectly accurate. She also learns that her classmates have leveraged their repeated experiences to master various skills, from academic knowledge (creating "past exam questions" from 12 loops) to languages and physical techniques. However, they must moderate their performance to avoid suspicion, as a past incident of perfect test scores led to Shiratori-sensei being accused of facilitating cheating.
The class's primary goal is to escape the loop, and they believe Natsumi's unprecedented arrival is the "dramatic change" that might finally allow it. Natsumi explains her transfer was due to her mother's job in Dubai, and her father being a day trader. The class speculates on how her existence differs from previous loops, ultimately deciding to focus on the changes her presence brings. They intentionally withhold future knowledge from Natsumi to preserve the "newness" of the year, assuring her that previous loops were generally peaceful.
Natsumi observes her classmates' unique skills during a health check-up, where female students like Yuzawa (track and field) and Narumi (archery) demonstrate honed "presentation" techniques and physical prowess, despite their bodies resetting each loop. She also learns about the "International Social Studies Research Club," led by Kakimoto, where classmates have mastered multiple languages. Natsumi joins on a trial basis, intrigued by the prospect of learning Arabic. She notes the intense dedication and high motivation of all loopers, who strive for mastery in their chosen fields, from sports to video games.
Andou reveals that the class wasn't always so harmonious; early loops were chaotic, with some classmates becoming self-destructive or engaging in extreme behaviors (like Kakimoto stealing money to travel). Around the eighth loop, they entered a "sage mode," realizing the futility of fighting and fostering a strong sense of camaraderie. Natsumi, though still unsure of her role, finds comfort and belonging with her new friends, enjoying their outings and the sense of community.
A trip to a newly opened karaoke bar reveals another loop-derived skill: Matsusaka and others have meticulously "restored" an unreleased hit song by the band Fortuna, including its copyright code, through repeated listening and analysis over multiple loops. They plan to sing this song for the autumn cultural festival. Natsumi discovers she can contribute significantly by playing piano accompaniment, a role previously unfilled. Her classmates are thrilled, seeing this as further proof of her positive impact and hope for breaking the loop.
Despite the underlying uncertainty of the loop and the possibility of being reset, Natsumi feels happy and secure in her new life, having found friends and a sense of purpose. She even expresses a desire for a 14th loop if it means staying with her newfound companions.
