It's the g-d d--m zombie apocalypse. Four men, paramilitary types, are being attacked by another group of people. The men have retreated into an enclosed courtyard in the bad section of town. They hide behind concrete barriers, burned out cars, and assorted city junk. The people attacking them appear to be normal looking in every way. They don't act like zombies, at least not yet. They zombies are alert, intelligent, and persistent. The way the men know something is definitely wrong about them is that they can be shot many, many times before going down. The zombies attack the men with their own guns while getting overhead direction from a helicopter gunship.
The human men are under siege, but fight back hard. Gary, the leader of this group of four, shoots at the helicopter with his AK-47. He stands up from behind his cover, yells as he shoots, and the helicopter eventually goes down spinning. Still the zombies are coming. The men shoot one zombie twenty or more times, but he still advances with murderous menace.
Gary calls for a retreat as more zombies-who-don't-look-like-zombies pour into the courtyard. They retreat into an area of stacked container cargo boxes next to a railroad car which is buried partially into a hill. They quickly set up barbwire and booby traps. As the zombies enter the enclosed space, the men shoot them down in a hail of metal.
One of the zombies drops an odd looking device. It looks like a metal green drinking glass with a needle like point in the center. It has a rugged plastic bottom that can twist into a locked position. The men soon discover that this is a type of grenade. When it explodes, it releases an intense blue light that is somehow more harmful to the zombies than to themselves. They recover a few of these grenades from the zombie bodies, and using them, they stop the seemingly endless zombie advance. Relaxing as much as they dare to, they fall asleep from their entrenched position.
They are awakened by a group of five men and women. They point the guns at this group. Mike, an imposing black man of considerable height and muscle, and the leader of this new group, convinces Gary that he and his friends are humans and not zombies. Mike explains that, actually, the zombies are not people either, but they are an invading force of aliens. If the aliens do not get their nutrients by drinking some kind of milk, they start to act like brainless zombies. It is something about the earth environment that causes them to act this way. Hence, the confusion. Gary and his group of four men have been isolated for a long time since the fighting began. He hasn't gotten much news and this new information makes as much sense as anything.
Mike and Gary become an alien fighting team. They learn how to navigate through a world of zombie-like aliens, even as the aliens adapt to their new environment. Several months pass as Gary, Mike, and their band fight the invaders. The aliens have developed into a significant invasion force, entrenching themselves in the cities and building up considerable support resources.
The aliens can only appear like normal human beings for three or four days, after which they become the brainless aggressive types seeking to kill any human they come across. For some reason, the noise emitted by vacuum cleaners will placate the alien zombie aggression, like music soothing the savage beast. They brainlessly push their vacuum cleaners around until more clear-headed aliens come along and feed them their milk nutrient to restore them to clarity.
Mike and Gary discover this odd fact. After killing one of the vacuum cleaning aliens, they grab its vacuum and push their way into the city. The come across a zombie/alien super market. One of the clear-headed aliens, a sergeant-major type, impatiently directs Mike and Gary, who are faking the brainless shuffle of a zombie, into the store after giving them passes. Mike and Gary come to the center of the store where there are large shelves filled with foodstuffs, including the new milk nutrient mixture that is more effective at staving off the zombie effect on the aliens. They grab a couple of three-gallon sized jugs of it, planning on studying what it is about the nutrient mixture that the aliens need. They are in some significant danger by being this brazen with their raid. They know that as they begin to shuffle out of the store and back out onto the darkened streets.
And then I wake up.