The Demon Queen Sheila has massacred over half of the human population, holding no regard for life. With no end in sight to her reign of terror, the people elect a hero to vanquish the Demon Queen and her monster kin. However, the chosen hero, Exa, is not exactly humanity's champion: he values not only human lives but those of monsters as well.

During his journey, the idealistic Exa saves a monster girl from a dragon, completely unaware that this girl is the Demon Queen herself. Intrigued by a human who would save one of his species' enemies, Sheila poses as a meek, lovestruck monster and joins him on his journey—all while planning to stab him in the back. Even as she is continuously exposed to Exa's ideals, Sheila stands firm on her plan. Yet, she finds herself putting off her original mission time and time again; perhaps these fake feelings have grown into something real...