I Really Don't Want to Save the Common People

A hundred years ago, animals and plants mutated, the world order collapsed, and humans were reduced to the bottom of the food chain. Over the past hundred years, humans have found ways to mutate, gained the ability to resist, and built barriers. The situation has been slightly reversed, but the people inside the walls seem to have forgotten the threat of mutant beasts over the course of time. A hundred years later, a young man walked out of the ruins and unknowingly changed the world.