The history of people building walls and committing themselves to them dates back hundreds of years, when a meteor pierced the sky. The dark night was lit like day by the fire generated by the friction between meteorites and the atmosphere, and with a loud bang, the world ushered in the greatest disaster in history. The space virus brought by the meteorite has infected most of the world's animals and plants, causing them to mutate into alien beasts. Humans were initially unaffected and did not mutate, but they were threatened by the alien beasts and became food for the beasts, and the humans were almost slaughtered. Some of the survivors have built walls to defend themselves against the beasts, believing that one day they will reclaim the land that belongs to humans.