Everything seems normal for Sion till, on the evening of Sion's twelfth birthday, he meets a boy who calls himself "Nezumi" (Rat) 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.