Sion is intelligent, highly educated and lives a privileged life within the walls of No. 6, one of six city-states that was built after the world was destroyed from war. But Sion's comfortable existence changes forever when he meets Nezumi, an escaped convict, on the eve of his twelfth birthday. Due to his assistance in keeping Nezumi safe, Sion loses his rights and is forced to live in the lower town, where he becomes a normal citizen. Four years later, Sion finds himself back in Nezumi's company and in the midst of a dangerous conspiracy that leaves a trail of bodies in its wake. What is really going on in No. 6?