10 years ago, Toneriko Furoribanda lost his entire family to the deadly tactics of Tekirai, the elusive "Number 1" hitman of Thanatos, a notorious guild of assassins. Blaming Tekirai for the tragedy, Toneriko surprisingly joins Thanatos and becomes one of the highly-ranked "Numbers" himself. Despite achieving the title of "Number 5," Toneriko is still haunted by his desire for revenge against Tekirai, and he longs to bring his family back to life.
Fortunately, the Numbers' overseer, Ageha, reveals the legend surrounding Thanatos. Upon completing 999 murders, a "Black Sheep" cycle is triggered for the 10 Numbers. After 10 Black Sheep cycles and 9999 murders, the current Number 1 ascends to Asgard and is granted immense powers from Yggdrasil, allowing them to rewrite reality to their whim. As Toneriko learns of this incredible possibility, he sets his sights on challenging Tekirai for the top spot, hoping to finally exact his revenge and achieve the impossible dream of restoring his lost loved ones.
