This leads to a mass invasion of the cottage by the raging maniacs outside; as Don, Alice and the boy scramble upstairs, he gets out the window first-and leaves the other two behind as he runs hell for leather across a field, barely making his getaway in a motorboat.

Twenty-eight weeks later, the U.S. has sent troops to London; headquarters are on the Isle of Dogs, a peninsula in the Thames in eastern London. It's covered with a Singapore-like cluster of tall new buildings, and a neat little trolley ferries in newcomers. These include Tammy (Imogen Poots) and Andy (Mackintosh Muggleton, an even better name than Imogen Poots), the children of Don and Alice; she's in her late teens, he's about twelve. Don greets them-there are few children in the group of survivors and military newcomers-and shows them their new apartment. They also meet Scarlet (Rose Byrne), a dedicated medical scientist attached to the unit. And we also meet soldiers Doyle (Jeremy Renner) and Flynn (Harold Perrineau, from “Lost”), the latter a helicopter pilot.

The two children are disturbed by their father's tale of their mother's terrible fate. Though everyone is supposed to stay put on the Isle of Dogs, the two kids sneak away and seek out their former home, pursued by authorities. To the shock of Tammy and Andy, they find their mother, terrified and quaking, alive in the house.

Scarlet learns that Alice's blood carries antibodies to the virus, which keep it from affecting her. Perhaps a cure can be found-but Alice has to be kept away from everyone else; while she may not be affected by the disease, she's a carrier. A contrite Don, torn by guilt, sneaks in to see her where she's strapped to a gurney, and he kisses her. To his horror, he realizes he's contracted the plague-but in a few seconds, he's a snarling, frothing berserk lunatic.

Naturally, things get much worse from here out.

Danny Boyle and Alex Garland, principally responsible for “28 Days Later,” hired Spanish director Fresnadillo to create a script with Rowan Joffe, Jesús Olmo and Enrique López Lavigne. They wanted to avoid the structure of the first movie, but still pit fleeing survivors against raging, murderous lunatics (who tend to drool blood). “28 Weeks Later” is very well structured, starting with a terrifying action sequence, calming down, then unleashing the terror again-and against all odds, keeping the tension high and the scares frequent.

There are several segments: first, there's the outbreak on the Isle of Dogs as the plague swiftly passes from person to person, and everyone tries to get off the peninsula. Horrified but dedicated snipers, including Doyle, try to spot the infected among the fleeing people, but can't do it, and so finally open fire on everyone. In the melee, Scarlet and Tammy flee after getting separated from Andy, but they're reunited later on.

Having left his post in fear and despair, Doyle guides them and a few others across deserted London, unaware that the infected Don is following them. Doyle radios Flynn, still in his chopper, trying to set up a rendezvous. But the infected drive them onward, through streets, into a grassy area (with a carousel), finally down into the Underground tunnels under the streets. Fresnadillo keeps the scenery varied and the tension blistering, with occasional pauses to allow his cast (and audience) to catch their breaths.

I found the ending regretful but probably inevitable.

The most annoying aspect of the movie are the several choppy, rapidly-edited sequences: the shots last just a few frames each, everything is hand-held and mostly moving, it's all intense closeups and cut so swiftly it's impossible to keep your bearings. It crumbles into fractured chaos, loses all narrative drive and most of the suspense. This technique is one of the most over- and badly-used styles in modern films. One of the few who used it right was Paul Greengrass in “The Bourne Supremacy,” but here it's merely an annoyance.

It would be less annoying, of course, if the movie was WORSE than it is, but “28 Weeks Later” is so well done, with such a strong cast, that these clumsy, trendy deviations damage it more simply because you do care about what's going on. These fragmented sequences take you instantly out of the movie, and that's the last thing the production team should seek to do.

Fresnadillo is generally so accomplished that the film largely survives these irritating sequences. He gained an international reputation with “Intact” of a few years ago; it had limited U.S. release, but Danny Boyle was impressed by it. Fresnadillo is aided by the well-structured script, which measures out action and slower-paced suspense scenes deftly and consistently from beginning to end. “28 Weeks Later” isn't the small gem its predecessor was, but some will prefer it because it's a more traditionally-structured movie, with bigger set pieces, than “28 Days Later.”

Despite its technological accomplishments-near the end, there's an apocalyptic sequence in which the streets of London become valleys of fire-the movie is basically raw and rough, unsparing of its cast and the audience. It takes reasonably steely nerves and a certain kind of trust to get through the movie; its sharp edges are likely to flay the unwary. But it's an exciting, frightening action/horror thriller, a good lead-in to summer movie-going.