Koyomi Takasaki enrolls in a college-preparatory institute following his parents' separation in a world where transitioning between almost identical parallel dimensions is commonplace. Despite his academic concentration and reserve, Koyomi finds it challenging to create any bonds. Nonetheless, this all changes when Kazune Takigawa, a fellow classmate, unexpectedly approaches him, claiming to have come from the 85th universe where they have a deep-seated romance.