It's a nightmare. You are trapped in an abandoned mine with an injured comrade at your side against God-knows-what. Make your stand, get out alive. Three words of advice: shoot to kill. These things are not human, they are monsters..
Try to escape a city that is infested with flesh eating zombies. Find new weapons, upgrade them and kill hundreds of zombies in 10 levels and 3 challenge levels.
Survive the night fighting off zombies from your barricade. Survive longer than a day by making use of the daylight hours effectively by repairing your barricade, searching for weapons and looking for other survivors to help repair and defend.
Kill all zombies before they reach the top of the canyon and wreak havoc on the citizens of Zombville. Kill them all, make money, upgrade your weapon and buy power-ups. Good luck!