In the distant future, on the outskirts left behind by the rapid concentration of advanced science and technology in urban areas, lies a remote mountain village known as Kagami Village. "Behave yourself, or a Double will come for you." Such old folklore still lingers in this village. Akane Higure, a middle school student living in Kagami Village, has been eagerly awaiting the return of her older sister, Sui Higure, who is finally being discharged from the hospital after a year. However, not long after her sister comes home, Akane receives a single phone call. "The girl at home with you… isn’t your sister."