Toneriko Furoribanda is an orphan whose family was slaughtered 10 years ago. He blames his mother and sister's deaths on Tekirai, the mysterious "Number 1" hitman belonging to the assassins' guild Thanatos. Contrary to reason, Toneriko does not hold Thanatos itself responsible, as he himself joins the guild and becomes one of the "Numbers" with a top 10 ranking. He quickly attains the title of "Number 5" but lacks the skills to climb any higher.

Toneriko longs to usurp Tekirai and exact his revenge against the man, almost as much as he wishes that he could bring his family back to life. The latter should be impossible, but the Numbers' supervisor, Ageha, tells Toneriko about the legend behind Thanatos' existence: each time the 10 Numbers execute 999 murders, a "Black Sheep" cycle is completed; if Black Sheep occurs 10 times, fulfilling a goal of 9999 murders—then whoever is Number 1 at that moment will be raised to God's Realm of Asgard and granted supreme power from Yggdrasil, able to rewrite the world as they desire!