In 20xx, the apocalypse broke out, and zombies only seen in novels and movies appeared. They swarmed in groups and ransacked every corner of the city. Except for the lucky ones who were discovered, most of them either became their companions or were cut into pieces. Cities fell one after another, and the number of humans dropped sharply. The government and the army began to build safe zones to protect the survivors. High city walls isolated the zombies, and the survivors within the walls temporarily had a place to live.