Many years ago, following the conclusion of a bloody world war, humanity sought refuge in six city-states that appeared peaceful and perfect... at least on the surface. However, Shion, an elite resident of City-State No. 6, gained a new perspective on his world thanks to a chance encounter with a mysterious boy named Nezumi. Nezumi turned out to be just one of many living in the desolate wasteland beyond the walls of this supposed utopia. Despite knowing the other boy was a fugitive, Shion decided to take him in for the night and protect him, which resulted in drastic consequences: due to his actions, Shion and his mother lost their elite status and were relocated elsewhere, and the darker side of the city began to reveal itself. Now, long after their life-altering first meeting, Shion and Nezumi are finally brought together once again—the former elite and the boy on the run are about to embark on an adventure that will, in time, uncover the shattering secrets of No. 6.