The world we knew had ended, the surface had been affected by a deadly virus that mutated animals into monsters wreaking havoc but then humans decided to give up on the surface and went underground and using cutting edge technology they developed intricate underground paths and built cities, but there was a problem these cities could only hold so many people and the rich took this opportunity to leave the poor on the surface, but what they didn't was that humans would eventually adapt to this environment and evolve in strength Now five hundred years later a huma boy named Caeul who loses his older sister to the mutated animals must learn to survive in this god forsaken surface as he tries to find a way to fulfill his dream to go to the underground kingdom of humans