Yuru is a boy who lives a quiet life in a remote mountain village, hunting game and enjoying the peace of nature alongside the locals. The only strange thing about his existence is his twin sister, Asa, who is forced to live a life locked away in a prison in the center of the village to fulfill a mysterious "duty." But this fragile illusion of serenity is shattered in a single night of pure terror: an army armed with helicopters and modern firearms raids the village, mercilessly massacring the population. Amid the carnage, Yuru discovers that his entire life has been a lie and that he and his sister are bound by a thousand-year-old prophecy. To save his own skin, the boy awakens the "Tsugai"—two powerful demonic entities called Right and Left—with whom he makes a pact to fight. Thrust into the modern world, a place he doesn't understand and dominated by ruthless warring factions, Yuru must learn to master these warring spirits, uncover the truth about his true family, and find the real Asa before darkness swallows what's left of his world.