Will Smith plays a military virologist who has inexplicably survived a man-made virus that wiped out mankind in the third or fourth film -- depending on what you count -- based on a 1954 Richard Matheson science fiction novel. But the film can never quite decide whether it's speculative fiction or a B-movie horror show.

"I Am Legend," the third or fourth film -- depending on what you count -- based on the 1954 apocalyptic science fiction novel by Richard Matheson, nails the emotional core in Matheson's story: What would it be like to be a last man on Earth?

Will Smith is, seemingly, that man: Robert Neville, a military virologist who has inexplicably survived a man-made virus that wiped out mankind. All of Manhattan is his home where he exercises, patrols and hunts with his dog Sam by day. At night, he hunkers down in his Washington Square townhouse with memories, videos and old albums that recall a vanished civilization. Vegetation and wildlife have reclaimed the eerily quiet canyons of gleaming yet useless high rises. The sight of Robert speeding in a car through such familiar streets, otherwise empty of human life yet littered with the debris of a populace that fled deadly bacterium, produces genuine shock.

Akiva Goldsman, rewriting a screenplay by Mark Protosevich for a project originally going to be made in the 1990s with Arnold Schwarzenegger starring and Ridley Scott directing, has made intelligent updates and revisions on this half-century-old story. But the writers retain the vampire element inherited from the literary source. Thus, the film can never quite decide whether it's speculative fiction or a B-movie horror show. It's not a fatal flaw, though, as "Legend" will be one of the most commercial holiday releases.

It seems the virus, developed initially to combat cancer (Emma Thompson puts in a brief cameo as the guilty inventor), not only wiped out most of the Earth's population but also caused severe mutation among survivors. They became vampire/zombies that shun the light but venture into the night to eat flesh. These humanoids made by CGI and motion-capture technology are annoyingly fake creatures that add a risible element to an otherwise overly serious epic.

As dusk comes each day, Robert boards up his abode, which is heavily fortified. Now 1,000 days into his ordeal -- the year is 2012 -- he broadcasts daily radio messages in search of fellow survivors. He also hunts and traps these "Dark Seekers" to experiment on their bodies with his immune blood to find a way to reverse the virus' effects. Then one day the creatures turn the tables: They trap him. He is wounded, and Sam gets bitten by plague-carrying dogs.

The third act either ups the ante of action and suspense or falls apart, depending on one's taste in science fiction. A young woman and child (Brazilian actress Alice Braga and Charlie Tahan) suddenly appear out of nowhere. A Judeo-Christian theme gets introduced, the creatures lay siege to the townhouse, and Robert discovers the antidote for the plague. All of which leads to an upbeat ending few are going to swallow.

Some viewers also might wonder at peculiar phenomena in this post-apocalyptic world: All utilities work perfectly, which might come as a shock to New Yorkers who experience problems with water, gas or electricity when a full work force mans those departments. While Robert's race through empty streets is visually thrilling, what's his hurry? And how did he single-handedly build a state-of-the-art lab in his basement?

Smith, sporting a newly buffed physique, delivers an extraordinary performance as a man slowly coming unglued under the strain of no human contact and a constantly alternating role of hunter and prey. The action and suspense do quicken the pulse under the assured direction of Francis Lawrence. A video director with one previous film credit -- the intriguing 2005 sci-fi'er "Constantine" -- Lawrence is in complete control of his actors, stunt people and visual effects. These, when blended with Naomi Shohan's knock-out production design, Andrew Lesnie's mobile camera and James Newton Howard's magisterial score, create a New York City that is a literally an urban jungle.