Angela and her cameraman, Pablo, are documenting the overnight shift at a fire station in Barcelona. Just when it seems as though the entire night will pass uneventfully, the station gets a call. It isn't a fire but a crazy old cat lady (according to her neighbors' description) is in distress and the fire department is needed to break into her apartment to rescue her. Naturally, Angela and Pablo go along. When Manu the fireman and his partner break into the apartment, they find the lady, Ms. Izquierdo, staggering around as if delirious. She's covered in blood, although seemingly not injured except for some scratches on her face and arms. Oddly, they don't find any cats in this "crazy old cat lady's" apartment. Hmm ...
A couple of cops arrive to take charge. The Ms. Izquierdo suddenly goes from being delirious to being frenzied. The senior officer gets his throat about half torn out. Manu's partner barely manages to wrestle her to the ground. Manu and the younger cop decide the situation calls for some backup. Right about that time, a SWAT team (or two, or three) surrounds the building. Yay! The day is saved. Or is it? Everyone inside is now under quarantine and they will be shot if they try to escape.
As you might guess, everything just gets worse from there. Ms. Izquierdo's rage virus is contagious, of course. So there's the cop she bit to worry about. While he's tying up the old lady, Manu's partner gets most of his face chewed off and tossed down the stairs by another infected (apparently Ms. Izquierdo's granddaughter or niece or something). So that's two crazies and two potential threats. Oh, and little Jennifer has been feeling sick, too, ever since her dog Max bit her while they were taking him to the vet ...
One of my favorite things about this movie is the dog is what tips off the authorities. The CDC doc (or whatever the Spanish equivalent is), explains when he comes in to take blood samples and administer heavy sedatives to the bitten. While Max was at the vet, he suddenly went feral and had to be put down. The vet naturally thought "rabies" and did some tests. It wasn't rabies, exactly, or anything the vet had seen before, but it was bad and it was communicable to humans. He called the Higher Authorities and that's why the building is now quarantined.
That's great for the world at large, not so much for our victims. Can a rookie cop, a fireman, two old people, an immigrant family, a mom with her sick daughter, a reporter, and a cameraman pull together and survive against a couple or four people infected with rage disease? Probably, sure. Now what if I told you it wasn't just a disease?
Ms. Izquierdo, for instance, gets up not once but twice from what were surely mortal wounds. Look closely during the big chase at the end, and you'll see the rookie cop up and running about despite having a (probably self inflicted) gunshot wound through his head. And, when Pablo and Angela lock themselves into the disused penthouse in a last-ditch effort to hide, they find a creature there that just simply CANNOT be explained as the product of a mere disease.
There are a few problems here but nothing especially serious. What I think would get on most b-movie veterans' nerves is the part where the cop confronts little Jennifer, who has by then succumbed to the infection. He's seen enough by then to know how dangerous the infected are. Hell, he just watched Jennifer bite off most of her mother's nose! Also, I think I can pinpoint the exact moment they were almost out of money and started cutting corners to get done before they got kicked out of the shooting location.
But that's nothing to really get worked up over. Images like an old lady shuffling feebly away from rage monsters while the able-bodied run off and leave her, Jennifer sneaking out of the dark behind the rookie cop, and the thing in the penthouse groping blindly for Angela, illuminated only by the night vision of Pablo's camera, are images that will come back to haunt you when you least expect them. That's the stuff a great horror movie is made of.