The year is 2013, and it's the city of No. 6: a stringently controlled, diligently maintained megalopolis that embodies the highest ideals of humankind. Sion has lived under close supervision as a "high-ranking elite and therefore one of the city's assets" since the age of two, and feels secure - if not particularly happy - in his carefully planned out, government-sponsored future.

On the night of his twelfth birthday, his comfortable lifestyle is turned upside-down when he meets, tends the wounds of, and shelters, a strange boy with an icy demeanour who calls himself "Nezumi" (Rat).

Why did I open the window that night? If I had not, I'd never have been forced to know what it's like to fight, to starve, and to suffer in anguish...