Oni Tensei follows Detective Reiko Kure as she investigates a gruesome mafia massacre where thirteen men are torn apart and only one survivor, the quiet Ema Nozomi, remains at the scene—her back marked by a vivid, lifelike demon tattoo. As Ema is placed under protective custody, those left alone with her meet a terrifying fate: every man is brutally killed while every woman is violently assaulted, drawing Reiko into a dark supernatural mystery tied to an ancient legend that a perfect tattoo can awaken a living oni.