In the aftermath of a devastating war, humanity sought refuge in six seemingly idyllic city-states. Among these, Shion, a privileged resident of No. 6, had his world transformed by Nezumi, a mysterious outsider. This encounter exposed Shion to the harsh reality beyond the city's walls, leading to a fateful decision to shelter the fugitive Nezumi. This act of kindness cost Shion his status and opened his eyes to the city's hidden horrors. Years later, their paths converge once more, setting the stage for a joint journey that will unveil the unsettling truths lurking within No. 6.