From the world of Gil Bok-sook, there is "Mantis," a top-tier assassin who comes back to a chaotic and disordered hitman industry after his vacation. As he reenters this anarchic environment, Mantis encounters Jae-yi, his fellow trainee and rival, and Dok-go, a retired legendary killer, and discovers that they are all competing for the No. 1 spot among the killers.