From Blu:
It's present day, and Archangel Michael is fed up with the pathetic development of the human species. The angels, in their world high above ours, have been given the choice between destroying or preserving humanity. This will be decided by having "Plus Checkers" looking for good points and "Minus Checkers" looking for bad points, and whichever finds 10,000 points first decides the outcome. Chihaya and Kagetsuya are angels who have known each other since their days at military school in Eden, and having taken up opposing sides of this "good vs. bad points" checking system, are teamed together during their stay on Earth.

However, Chihaya is unusual even by angelic standards—he has black hair, eyes and wings. He's one-of-a-kind, unusually powerful in spite of his naive demeanor (much to Kagetsuya's dismay), but set apart from his angel kin...until the Black Cancer strikes the heavenly population, darkening the wings and hair of its victims before spiraling them down into painful death. Tension builds among the angels and between Chihaya and Kagetsuya in particular, and it looks as if the secrets to the salvation or destruction of both Earth and Eden may lie with Chihaya himself.

Note: The English version is of the Japanese Deluxe rerelease, contained in only four volumes and also having some bonus materials.