Following a devastating world war, humanity found refuge in six city-states that appeared to be idyllic paradises. However, beneath the surface, these cities were not as perfect as they seemed. Shion, a distinguished dweller of city-state No. 6, came to this realization after encountering Nezumi, a mysterious boy who revealed the existence of a barren wasteland beyond the city walls. Despite knowing that Nezumi was a fugitive, Shion offered him shelter for a night, an act that led to severe repercussions. Shion and his mother lost their privileged status, and the dark secrets of No. 6 were unmasked. After years apart, Shion and Nezumi are reunited and set out on a journey that will expose the shattering truths behind the facade of No. 6.