In a world where humans and monsters have warred for years toward mutual extermination, the surviving humans name Exa their Hero and send him to kill the Demon Queen Sheila. Exa accepts the quest but refuses to slaughter monsters indiscriminately, convinced they are living beings like anyone else. Intrigued, Sheila joins him in disguise, meaning to win his trust and kill him, and travels at his side without letting on that the queen he is hunting is walking beside him.