In an age of ruin, humanity lives in dread of those marked 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 this endless cycle of hatred, a young boy and girl appear in a distant town, offering to bury an undead creature 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?