Several decades back, following the conclusion of a bloody world war, humanity took refuge at six city-states which were tranquil and ideal… at least on the surface. But, Shion–an elite resident of this city-state No. 6–obtained a fresh outlook on the planet he lives in, as a result of a chance encounter with a mysterious boy, Nezumi. Nezumi was be merely one of those who dwelt at the desolate wasteland beyond the walls of their supposed utopia. But despite understanding the other boy was a fugitive, Shion chose to take him for the evening and shield him, which caused extreme consequences: due to his activities, Shion and his mom lost their standing as elites and were hauled elsewhere, along with also the darker side of town started to make itself understood.Now, a very long time following their life-altering first assembly, Shion and Nezumi are eventually brought together once more –that the former elite and the boy on the series are just about to embark on an experience which will, over time, show the shattering secrets of No. 6.