The year is 2013 in the city of No. 6, the city that embodies the highest ideals of humankind. Shion has been raised here as a high-ranking elite since the age of two, but on the night of his twelfth birthday his fate is turned upside-down when he meets a boy who calls himself Nezumi.

"Why did I open the window that night? If I had not, I'd never have needed to know what it is like to fight, starve, or suffer in anguish...."

(Source: Nostalgia on 9th Avenue)