Jacob Hansen, second lieutenant, has the unfortunate luck to stumble right into the zombie armageddon. To make matters worse, his Captain is no longer available and so he has to take control over his platoon. The story is told between the campaign missions through static pictures (MS Paint style) and voice acting.
Zombie Armageddon is a tower defense game. The basic goal of each mission is to keep the (in waves) approaching zombie horde away from headquarters. This is done by placing troops (rifle, grenadier or tech squad) on rooftops along the way so they can kill the zombies on their way. The different troop types (which can also be upgraded once placed) have different efficiency against certain zombies. In order to deploy or upgrade squads, the army Humvee needs to drive to the building in question and back to the HQ before it can start on the next assignment. This means the player needs to weigh where to go next because the Humvee can't just drive through the main zombie horde without being destroyed. Other options are upgrading the Humvee (speed, weaponry and armor), laying barbwire on the road to slow the enemies down and destroying buildings to open up more ways.
All this costs money which is earned by surviving waves and rescuing civilians (which has the price of losing the Humvee for the time it takes for the rescue). The height of buildings also plays a tactical role. Besides the campaign, there is an onslaught mode which features endless waves on random maps.