In the professional killer guild known as Thanatos, the top ten assassins are ranked in a brutal hierarchy and referred to as Number Dogs. The protagonist is Toneriko, the current No. 5, who is driven by a desire to change his fate and claim the No. 1 position. His motivation is deeply personal: the murder of his family by the current No. 1, Tekirai, has set him on a path of vengeance. The story begins as Toneriko bares his fangs in pursuit of the top rank, setting off a bloody conflict that threatens to tear the guild apart from within. Early in the narrative, Toneriko clashes with No. 4, Sakura, but their fight is interrupted by an unexpected third party. As Toneriko approaches what seems to be his final chance to reach No. 1, a mysterious rebel faction called the Mitsubachi (Honeybees) infiltrates Thanatos, revealing strength that rivals the guild's elite assassins. This intrusion ignites a full-scale, blood-soaked war between the Number Dogs and the Mitsubachi, with Toneriko caught in the middle. In the conflict's climax, the formidable No. 2, Kuchinashi, finally appears, displaying overwhelming power. Toneriko pursues the Mitsubachi's leader, Suito, after she is taken, only to find that No. 3, Araragi, and Edelweiss have arrived ahead of him. The ranking battles, designed solely for promotion through killing, ultimately reach their end. The series concludes with the final fate of Toneriko as the Number system and the guild itself face their terminus.