Restless, irksome, strange: There’s not much downtime in Danny Boyle’s new film. From its first moments, when a crew of ski-masked, PETA-style activists break into a London lab to save test animals, the camera is in motion, the cuts convulsive, the shadows ragged. As they approach the cages, a lab-coated doctor type tries to stop them. The chimps, he blurts, are "infected." With what?, asks one girl, prone to tears at the sight of abused creatures. Comes the ominous, shaky voiced answer: "Rage." And with that, the teary girl unlocks the cage and the chimp leaps at her, sputtering furiously, ripping at her face as she screams.

Twenty-eight days later bicycle courier Jim (Cillian Murphy) is waking up in a hospital room. Ripping the tubes from his arms, he staggers down the hallway to find the place deserted, the ookiness boosted by Anthony Dod Mantle's grainy-digital, frankly hard-to-read video. He can only remember that he was in an accident on his bike, but can't even guess what's going on now. Wandering the streets, he looks out from Waterloo Bridge, picks up a weeks-old newspaper that reads "EVACUATION," and stumbles on a notice board at Piccadilly Circus: photos, scraps of paper, desperate pleas for help in finding the missing. (Shot before 9-11, the image eerily evokes NYC's memorials and notice boards.)

As Jim eventually discovers -- when he's discovered by a pair of survivors, Selena (Naomie Harris) and Mark (Noah Huntley) -- Britain has been decimated by the rage virus, passed on by saliva, blood and other bodily fluids. Once stricken, the victim has only a few seconds before he or she turns into the most spastic of zombies, filmed and edited to resemble some speed freakish nightmare, all flailing limbs and staccato movements.

Indeed, 28 days later... , directed by Boyle, written by Alex Garland and produced by Andrew Macdonald (the same team who made The Beach), is upfront about its inspirations (George Romero's Night of the Living Dead and The Crazies, John Wyndham's The Day of the Triffids, Geoff Murphy's The Quiet Earth, Thom Eberhardt's Night of the Comet, even the video game-based Resident Evil). Its maniacal mash-up sensibility extends to plot, which lurches from moment to moment and mood to mood. Bad, don't-go-in-there! ideas are obvious (Jim's insistence on seeing his certainly dead parents leads to brief nostalgic misery and lasting disaster), but seeming good ideas don't really hold up either.

Following some unsettling run-ins with roving, hyperkinetic infecteds, Jim and Selena meet a warmly welcoming cab driver, Frank (Brendan Gleeson), and his teenaged daughter Hannah (Megan Burns). The tender father-daughter dynamic differs crucially from Jim and Selena's flinty affinity; this at Selena's behest, as she knows you have to be able to kill an infected, no matter who it is, at a moment's notice. (Her own capacity for such violence is brutally demonstrated, a useful lesson for gentle Jim.) Selena's worry that the family will only slow them down speaks to the film's basic quandary: Is it necessary to lose traditionally "human" compassion, to match the infected's ferocity, in order to combat them?

This dilemma comes to a climax when the newly formed band leave the city in search of a "safe haven" outside Manchester, announced by a weeks-old radio recording. This turns out to be a military outpost commanded by one Major Henry West (Christopher Eccleston). The soldiers are ensconced in an abandoned mansion in the country. The dangers of infected assaults change shape -- in the city, they resemble gangs, so awful that rats run from them; out here, they're like marauding animals. Proud of his domain and looking to protect and maintain it at all costs, West shows off his cook's talents with eggs and his "experiment," an infected black soldier they've been able to chain up, now snarling and speed-heaving against his constraints, so they might observe him (think: frantic chimpanzee in a cage).

Here the film lays out another, less clearly worked-out concern about the ways that race frames the very concept of infection. Selena's blackness alongside everyone else's whiteness initially seems a nonissue, as the more significant difference is the infected's transmutation into a nonhuman race. The appearance of this black soldier, however, potentially (and however unintentionally) redraws distinctions and raises difficult questions, recalling the effect of Ben's (Duane Jones) blackness in 1968's Night of the Living Dead (where the lynch mobbish sheriff's posse drew attention to racism and civil rights issues).

Selena and Jim each undergo specific transformations in the film. She shifts from tough, smart action girl to distressed damsel (granted, this is overstated visually, as she's forced to don a gown, as if to suggest irony). And he becomes a vicious killer, as ready to exact ugly vengeance as any conditioned and ultimately desperate soldier boy. Both these trajectories are complicated and inexact, adhering to genre conventions but also strained and subversive. While the virus metaphor is obviously timely, the characters' seeming capacity to forget these nasty changes by story's end may be the films' most unsettling aspect.