Toneriko Furoribanda is an orphan whose family was massacred 10 years ago. He holds Tekirai, the enigmatic "Number 1" hitman of the assassins' guild Thanatos, responsible for his mother and sister's deaths. Defying logic, Toneriko does not blame Thanatos itself; instead, he joins the guild and becomes one of the "Numbers," securing a top 10 ranking. He rapidly earns the title of "Number 5" but lacks the ability to advance further. Toneriko yearns to overthrow Tekirai and exact revenge on him, nearly as much as he wishes he could resurrect his family. While the latter should be impossible, Ageha, the Numbers' supervisor, informs Toneriko of the legend behind Thanatos' existence: each time the 10 Numbers commit 999 murders, a "Black Sheep" cycle is completed; if Black Sheep occurs 10 times, achieving a total of 9999 murders—then whoever is Number 1 at that moment will be elevated to God's Realm of Asgard and granted supreme power from Yggdrasil, allowing them to rewrite the world as they desire!