Until Children of Men came along, Danny Boyle's 28 Days Later was this young millennium's most potent end-of-days fantasy. Released in the wake of September 11 and right around the time of the first SARS scare, the movie vividly imagines a first-world nation--in this case, England--plunged into chaos following an unprecedented disaster that decimates much of its population. (Of course, because it's also a horror movie, the dead are promptly reanimated...as rage-fueled, red-eyed, flesh-eating zombies.) Boyle instantly grabs the viewer's attention with a memorable opening sequence, in which a lone survivor (played by a then-unknown Cillian Murphy) wanders through an empty London encountering only looted storefronts, trash-strewn streets and pleas for help scrawled over photos of the missing. Much of this scene's power comes from the director's decision to film it guerrilla-style, using multiple digital cameras in actual locations with a minimum of props. The grainy images make you feel as if you're watching a home video from an alternate reality, one in which the apocalypse has already occurred.

Contrast that with the first glimpse of London we see in 28 Weeks Later, which picks up the narrative some six months after the events of the previous movie. Instead of shooting from street level with a handheld camcorder, incoming director Juan Carlos Fresnadillo straps his 35mm camera to a helicopter and takes us flying over the deserted metropolis. As beautiful as this God's-eye view is, it distances us from the setting in a way that the first film refused to. And in broad terms that's the primary flaw of 28 Weeks Later--it allows us to observe the action, but rarely makes us a part of it.

To the filmmakers' credit, we do get to observe some pretty great action during the course of the movie. Where Boyle doled out the zombie attacks sparingly, Fresnadillo doesn't hold back, opening with an undead assault on an isolated farmhouse inhabited by a dwindling number of uninfected humans. (Any similarities to >Night of the Living Dead are surely intentional.) Among the survivors are Don (Robert Carlyle) and Alice (Catherine McCormack), a married couple whose children were on a field trip abroad when the virus swept through London. After the zombies break through their defenses, Don abandons his wife to save his own skin, a decision that eats away at him each and every day thereafter. Making his way back to civilization, he enters a London that's now zombie-free courtesy of the U.S. military, which has taken it upon itself to restore law and order to yet another strife-ridden country.

While waiting to be reunited with his son Andy (Mackintosh Muggleton) and daughter Tammy (Imogen Poots), Don secures a job as a building supervisor in the city's equivalent of a Green Zone, where the first waves of returning citizens are watched over by a bevy of snipers and heavily armed foot soldiers. Also on hand are scientists like Scarlet (Rose Byrne), who is obsessed with finding the source of the virus and, if possible, a cure. Although her superiors believe the threat to be extinguished, she suspects that they haven't seen the last of this plague. It goes without saying that Scarlet's fears turn out to be justified. Through circumstances that are more than a little contrived, an infected human is discovered (to say who it is would spoil the surprise) and the virus is subsequently introduced back into the population. Learning that Andy may possess some kind of natural immunity, Rose attempts to get him and his sister to safety, just as the military higher-ups declare Code Red--a plan that calls for the wholesale slaughter of everyone in London, whether they're a zombie or not.

The last half of the film is an exhaustingly sustained battle sequence that owes more to James Cameron's Aliens than anything in 28 Days Later. The movie's grim intensity is its greatest asset; it's made clear early on that none of these characters is safe and, indeed, the body count is shockingly high. Fresnadillo isn't as skilled an action director as Cameron--the shaky camerawork and quick cutting often makes it difficult to follow who's shooting at (or biting into) whom--but he does know how to keep viewers perched on the edge of their seats with terrifically tense moments like a descent into a subway station shot entirely through the night-vision scope on a sniper rifle. Much will surely be made of the film's blatant political allegory, but to be honest, this material is only intermittently effective. As chilling as it is to watch American soldiers ruthlessly carrying out orders to execute innocent civilians, there's a heavy-handedness here that other socially conscious horror directors like George Romero and Joe Dante are usually able to avoid.

That 28 Weeks Later manages to be an exciting, if uneven, action/horror hybrid is all the more surprising when you consider that the movie has no reason to exist. 28 Days Later told such a complete story, it seemed that a sequel would only belabor--or repeat--what had come before. In both style and content, these movies are very different creatures and some viewers may emerge from the theatre preferring Fresnadillo's blood-soaked Judgment Day to Boyle's quieter apocalypse. In this critic's opinion, those viewers would be crazy, but hey, everyone has their own ideas about how the world will end.