More than two hundred years later , The Earth's environment has become so toxic and evolved in a distorted way that humans must wear a special full-face respirator mask to breathe. Without them, humans can hardly survive for two minutes. However, in some places, mysterious mutations have occurred, leading to the birth of Mutated Creatures. These places are known as Gene-Mutation Zones.
The Earth's atmosphere is covered in thick, grey haze. The sky looks dull and lifeless, with no sign of the blue it once had. The sun is weak and hidden behind the cloudy air, giving everything a cold, damp feeling. It's not freezing, but the chill is constant, making the world feel uncomfortable and dull, like a long, overcast winter day.