In an era of ruin, mankind lives in fear of those branded with the "Devil's Mouth"—immortals who are demonized, hunted and killed, only to return and exact vengeance on those who have harmed them. Amid the endless cycle of hatred, a young boy and girl appear in a distant town, offering to bury an undead that has been tormenting its people.

But who is this boy who bears the mark of their enemy?

Could they be the key to humanity's salvation?