The city is full of infested zombies.  The food are depleted in that area so they are looking for foods everywhere they go.  At last, they are in your town and they saw you eating some food.  They want your food and want it for themselves.  You ain't ready to give up on your food supply and would fihgt for them.   You need to defend your home from invading zombies.  Since you aren't prepared yet, you can use a stone to kill them.  Don't worry help will soon arrive and help you battle them out.  Each zombie you kill will earn you money.  You can use the money to buy weapons and other stuffs to help you in your campaign.