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.”

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