Nobody know what happened as it only take one night to change the entire village. You are sent in by the government to get rid of the zombies. The idea of this game is to make sure that none of the zombies are able to leave the area. You need to get rid of them by all means. You would unlock powerful weapons as you make some progress. Enemies would be tougher and their number would increase as you make some progress, but you don't need to worry since you can use more powerful weapons too. The game would get even harder as you make some progress.