Yuuna is just an average girl in this post-apocalyptic world, when one day an unexpected threat brings her hometown to ruin. As she leaves everything behind and sets out on a journey for her loved ones, the girl learns the secrets of her world, but also of her own origin and fate. Lucky enough to be taken in by a secret organization, Yuuna becomes a shadow that rids humanity of internal corruption caused by the demonic entities ruling over the lands. Will she succeed in purging the other-worldly evil?