A genre which has enjoyed another of its many revivals in recent years - '28 Days Later', '28 Weeks Later', the Ireland-shot 'Reign of Fire', 'War of the Worlds' - the post-apocalyptic movie has, at its best, the ability to frighten and fascinate in equal measure with the 'what ifs' it throws up on the big screen. Having secured the biggest December opening in US box office history, Will Smith's 'I Am Legend' now reaches European cinemas. But those expecting the traditional Christmas action-and-wisecracks blockbuster are in for a shock.

When a miracle cure for cancer goes awry in 2009, 90% of the world's population are killed outright by the virus, with the remainder turning into cannibalistic mutations that can only prey after dark. But there is one uninfected human: Robert Neville (Smith), a US military virologist who had been working on an antidote when civilisation collapsed.

Holed up in his heavily fortified New York townhouse, Neville searches in vain for other survivors and a way of making the cannibals human again. With only his German Shepherd Sam for company, he wanders New York in the daylight every day, searching through apartments and broadcasting a rendezvous point over the airwaves. He gets home by dusk and hides out until the next morning.

After three years, Neville is convinced he's the only one left in the world and the psychological trauma that results has pushed him to the edge. Is there any point in continuing with his work?

Based on Richard Matheson's classic book of the same name - as was the Vincent Price-starring 'The Last Man on Earth' and Charlton Heston's 'The Omega Man' - 'I Am Legend' has plenty to offer but the longer the story goes on the more you feel that it had the potential to be even more.

For the first hour it is excellent as Smith settles into the profoundly bleak and slow-moving storyline, once again revealing his depth as an actor and once again convincing you that he should do more serious drama. Because of the nature of the film, the role of Neville carries with it huge potential for grandstanding, but Smith's subdued performance as humanity's last hope is perfectly judged.

The visuals are sometimes stunning and it is the depiction of a ruined and empty New York which is far more memorable than any setpieces director Lawrence offers. We see Smith's Neville hunting for deer in Times Square, practicing his golf swing off the back of an aircraft carrier and visiting a DVD store where he tries to pluck up the courage to 'speak' to one of the mannequins he's put in the aisles to keep him company. The isolation is deeply affecting.

Where 'I Am Legend' has problems is that it switches pace far too quickly two-thirds of the way through. Having been in no rush with the events on screen, Lawrence starts to rush things and spoils the momentum he had built up. Too many things happen too quickly, there isn't enough time to address the developments and the ending is unsatisfactory because the clock has become such an issue. 'I Am Legend' is too short for the themes it was trying to deal with - a pity, because all the elements were here for a genre classic.

Unlike 'The Omega Man' this isn't a film that sci-fi and fantasy fans could watch twice in succession. But it is worth seeing.

Harry Guerin