The world as we knew it ended. Radiation covered the world aboveground and turned it into hell. And humanity headed deep into the underground. Using the world's most developed underground network as its backbone, they built an underground city. The last hope of humanity, the underground city of Shindorim. And we, who could not enter the underground city, were left behind in hell. So we had to get strong. And we did.

(Source: MangaDex)