It's been three centuries since the last human left Earth. Manipulated by sweet sounds slogans like saving Earth's environment, people immigrated into space colonies. After spending more than four centuries in the low gravity environment of space colonies, mankind evolved into a race that can no longer live on Earth.

With dozens of millions-class colonies bundled together, this megalopolis-class colony contains 150 million people, far over its design capacity and what is considered safe.

Countless illegal residents took to the building of illegal structures in the upper levels. New colonies are built to cheaper and cheaper standards, with cheap thin air and cheap reduced gravity.