Shion is a privileged boy raised among the elite of No. 6, one of the last city-states standing after war devastated the world. On his twelfth birthday, during a storm, he shelters an escaped prisoner who gives only the name Nezumi and treats his wound, an act that strips him of his status and sends him down to Lost Town. Four years later a parasitic wasp kills a coworker in front of him, Shion is arrested for what he has seen, and Nezumi reappears to pull him out past the walls.