No. 6 Sion is a bright teenager living a comfortable and promising life inside No. 6, one of this six remaining city-states created by The Babylon Treaty after the last great war devastated the world. On the rainy evening of his twelfth birthday, he meets a savvy adolescent who calls himself "Nezumi" (Rat) and is desperately trying to runaway from the authorities. For helping a fugitive of the state, Sion is stripped of all his privileges. Four years later, they meet once again. For better or for worse, Sion is about to unravel the secrets guarded deep inside No. 6.