Since the beginning of Earth's history, angels from the planet Eden have been carefully monitoring the behavior of its inhabitants, whom they call "Earthians." Chihaya and Kagetsuya are two agents sent from Eden to assess the positive and negative aspects of the Earthians' nature. Despite the orders Chihaya was given by his home planet, he frequently ends up entangled in Earthian affairs, much to Kagetsuya's annoyance. Meanwhile, as Chihaya tries to understand the origin of his strange black wings, he uncovers one mystery after another.