While searching for his precious friend, a young boy named Nai is captured by a beautiful woman, whose appearance is matched only by her taste for human flesh. Meanwhile, Gareki, a clever thief, is in the middle of robbing her luxurious home. After creating a distraction, Gareki agrees to help Nai escape, but they are discovered when the woman returns. As she transforms into a ghoulish monster, the boys flee. On the run, Nai and Gareki are found by "Circus," a government defense agency that handles criminal activity too difficult for the police to manage and protects civilians from "varuga"—terrible monsters that devour humans for sustenance. Hoping it will lead Nai to his missing friend, he and Gareki decide to join Circus. On their perilous journey, they face dangerous varuga and begin to uncover the secrets behind a shadowy organization known as Kafka.