It is 2013 in the city of No. 6, a place that represents the highest ideals of humanity. Shion has lived here as a high-ranking elite since he was two years old, but on the night of his twelfth birthday, his life is turned upside down when he encounters 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...."