In the exciting new horror film 28 Days Later, British director Danny Boyle (Trainspotting) imagines a fantastic situation - the fall of modern England to a disastrous blood epidemic that produces marauding zombies. He then subverts the familiar horror elements with a comparably savage vision of a safe haven possibly more terrifying than the chaos outside its walls.

The film starts off with a bang - a group of animal activists break into a covert laboratory in a misguided attempt to free a group of caged chimpanzees - only to discover the beasts to be diseased with "rage" and inadvertently release a deadly virus to end all others.

Skip forward 28 days and we meet young Jim (Cillian Murphy), awakening from a coma in a deserted hospital and a deserted London.

The film plunges into nightmare mode quickly, as he finds himself on the run from some very savage zombies, and into the protection of tough renegade Selena (Naomie Harris), a fellow survivor hiding out amidst the rubble. As they make their way through a burnt out shell of London, searching for survivors, protection and keeping bloodthirsty demons as bay, they encounter two more refugees; father and daughter Frank (Brendan Gleeson) and Hannah (Megan Burns).

Responding to a radio broadcast promising a safe haven somewhere out of London, they band together in search of sanctuary. When they arrive at their destination, they're relieved to find a temporary safe haven, a militant sort of police state as it were, overseen by Major Henry West (the fine Christopher Eccleston). West initially seems a rather level voice of reason and much-needed leader amidst the chaos. But there are sinister plans in motion that prove him a sadist on the order of his zombie counterparts.

There's a visceral pull to most of the film, particularly the unsettling and terrifying first half, as the lone survivors are hurled from one deadly, bloody scenario to the next. And unlike most Zombie films, including those of George Romero and Lucio Fulci, these rabid creatures move about quickly and furiously (with the manic gusto of Lamberto Bava's Demons), unexpectedly running, leaping and darting into the frame at surprise intervals. It's an unnerving experience, and the absolutely ironic claustrophobia of being trapped in an abandoned, wide-open city is mesmerizing. It's an awful nightmare, washed out, bleak and hopeless.

28 Days Later may be all the more terrifying in that Boyle had the absolute sense to shoot the film on digital video, heightening the in-your-face feeling of the film's many shocking and horrific showdowns and enhancing the realism of a city destroyed as if by war.

The digital format suggests a creepy, up-close reality that, for this material, seems the perfect choice in hammering home the immediacy of the terror. 28 Days Later, though impeccably made, isn't a particularly striking film from a cinematography point of view, and deliberately so. It's grimy, often dark, and when the zombies attack, chaotic. There's no real definitely stylization or flash, just energy, grain and grit.

The acting is uniformly fine, with one standout. As the tough yet tender Selena, Naomie Harris, an actress I've not seen before, pulls out the stops. Her performance is intensely physical, as she more than holds her own against the marauding and contagious pack, initially the group's most savvy protector. But her maternal, softer instincts for Hannah set in during the film's second half (think Sigourney Weaver in full-out Aliens mode), and the combination of guts and grace is striking.

I've heard some critics complain that the second half of 28 Days Later loses steam and gets a bit lost once the heroes enter the military compound. But for me, that's just when things begin to get more provocative and thoughtful. By the time a feral Jim, behaving not unlike the zombies themselves, unleashes hell on West and company, the film achieves a kicky, manic and bloody pitch, of which Dario Argento would be proud.