In the walled utopia of No.6, privileged elite Shion shelters a fugitive boy who gives only the name Nezumi, or 'Rat', on his twelfth birthday. Four years later a parasitic wasp infection that ages its victims to death spreads through the city, and Shion is arrested as a suspect. Nezumi reappears to break him out, pulling him past the wall into the slum known as West Block and the truth the city hides. This is Atsuko Asano's original novel series, not its manga adaptation.