The animal rights activists move through the darkened lab, dressed in black like ideological ninjas. As they head for the chimp enclosures, a researcher appears in their path, warning them that the apes are highly infectious and very, very dangerous. No matter: A cage is opened, a chimp is freed -- only to promptly sink its teeth into its liberator's face. Within seconds the woman is transformed into a raging, red-eyed psychopath and the room becomes a bloody chaos.
Twenty-eight days later, England is a country of corpses.
It's been a long time since we've had a good end-of-the-world zombie movie and, given the state of global affairs, now may be a discomfiting time to get one. Our tough luck, then: ''28 Days Later'' is a damn-near great end-of-the-world zombie movie, terrifying on the basic heebie-jeebie level, respectful toward its B-movie forebears, and all the more unnerving for coming out in this fretful era of SARS and germ warfare.
It also represents a filmmaking bounceback for director Danny Boyle and producer Andrew Macdonald, the team that gave us ''Trainspotting'' before being diverted with layabout Leo DiCaprio to ''The Beach''.
After the hellacious opening sequence has pruned the audience of the squeamish -- goodbye, don't forget to write -- ''28 Days Later'' wakes up in the hospital with Jim (Cillian Murphy), a bike messenger who has been in a monthlong coma following a traffic accident. In classic last-man-alive fashion, Jim wanders the deserted streets of central London, past the House of Lords and Big Ben, shouting hello to anyone who might hear. You really hope he doesn't get an answer.
But he does, and in a way that lets you know the movie won't be taking prisoners. Soon Jim is being chased by ravening crazies -- not zombies in the strict technical sense, his assailants are alive, infected, berserk with homicidal rage, and scarily fast on their feet -- and he's saved only by the appearance of two other survivors, Selena (Naomie Harris) and Mark (Noah Huntley). They lay out the ground rules: If you get infected blood or saliva on you, you'll go insane in less than 20 seconds, and your compatriots will kill you for their own protection. No questions asked, and no time to call your primary care physician for a referral.
Jim and Selena are joined by Frank (Brendan Gleeson, the hulking Monk McGinn of ''Gangs of New York'') and his adolescent daughter, Hannah (Megan Burns), and these four break out of London toward Manchester, drawn by a radio broadcast that promises military rescue. The journey is white-knuckle tense even when not much is happening: Director Boyle, using scrappy digital-video camerawork that either feels like notes from the afterworld or like tomorrow's nightly news, has primed us to expect the worst at any moment.
The military garrison, when it's finally reached, contains its own horrors, but they're of a more ordinary stripe compared to the brain-eating derangement of the earlier scenes. The soldiers, led by cadaverous Major West (Christopher Eccleston) are friendly to the interlopers -- a little too friendly -- and that's all I'll say about that.
''28 Days Later'' does wrap up with a satisfying freakout that allows Jim to get in touch with his own inner zombie, but there's been some damage done, and you start to see the plot implausibilities through the holes. (Why do the infected only come out at night? Why do many people appear to have died without becoming rageaholics?) It's also possible that the movie will seem overly gruesome to audiences who know only mainstream horror films like ''Scream'' while disappointing hardened gore-hounds who like their steak extra-rare.
Still, cut the movie slack for scaring an audience so blitheringly silly in the first two-thirds. Be grateful, too, that Boyle and company nod to all the apocalyptic pop that has come before: George R. Stewart's seminal 1949 speculative novel ''Earth Abides,'' the old ''Twilight Zone'' episode ''Where Is Everybody?,'' the 1971 Charlton Heston zombie flick ''The Omega Man'' (Harris's stoic Selena could be the daughter of that film's Rosalind Cash), and the work of schlock-movie geniuses like George Romero and Dario Argento.
''28 Days Later'' is a worthy, blood-flecked addition to the lineage.