Since the dawn of Earth's history, angels from the planet Eden have been closely observing the behavior of its inhabitants, whom they refer to as "Earthians." Chihaya and Kagetsuya are two agents dispatched from Eden to evaluate the positive and negative traits of Earthian nature. Although Chihaya receives orders from his home planet, he often finds himself drawn into Earthian matters, much to Kagetsuya's irritation. Meanwhile, as Chihaya attempts to understand the origin of his peculiar black wings, he uncovers one mystery after another.