Naive boy Nai has been looking for someone important to him, with only an old bracelet and the name "Karoku" as his clues to their whereabouts. Along the way, he becomes ensnared by a monstrous woman hellbent on keeping him as a pet. But soon, he is saved by a clever and crafty thief named Gareki, who has broken into the woman's house to loot it. The two find themselves labeled as wanted criminals and end up entangled in the affairs of "Circus"—the country's most powerful defense organization. Circus is responsible for protecting the nation's citizens from entities known as "varuga"—monsters who consume human flesh—as well as uncovering the operations of a mysterious organization called Kafka.

Circus has taken an interest in Nai upon learning that the bracelet he possessed turns out to be an old Circus ID. While Nai decides to stay in Circus' care, Gareki struggles to find his place aboard the ship. As their search for Karoku deepens, the pair encounter more dangerous varuga who all seem to gravitate toward Nai.