In the aftermath of losing his parents and living with relatives, high school junior Naoki Kuzumi leads a seemingly mundane life marked by a shroud of lost memories from his childhood. However, his routine takes a drastic turn when a red-haired girl, Mikoto, unexpectedly plummets from the sky and mistakes him for her sibling. Unbeknownst to Naoki, his era serves as a refuge for individuals from the future grappling with an untreatable illness. Determined to locate her brother, Yuusuke, who was transported to Naoki's time, Mikoto embarks on a journey through time itself.