"I Am Legend" is the third iteration of a 1954 novel by Richard Matheson, the story of a man, alone in a post-apocalyptic Manhattan, the survivor of a deadly virus that three years before seems to have killed everyone but him. Naturally he's not only immune, this time he's a military doctor who now spends his time working frantically at the lab in his lovely Washington Square home, trying to find a vaccine for the virus, barricaded against the rabid zombies who've been infected but haven't died; one bite from them and he too will die. Well, it's a setup, and not a bad one. As with most zombie movies, they only come out at night, when our hero must close all doors and windows until morning.

The hero's name is Robert Neville, and he's played by Will Smith, one of the few Hollywood stars who can not only do comedy - "Men In Black," for example - and also a brilliant paranoia - that neglected masterpiece "Enemy of the State" -- but has that marvelously elusive quality that only the great screen actors have of keeping our eyes and ears riveted on every move and voice and gesture we see on the screen.

The computer-generated effects of a deserted Manhattan are brilliant, from grass growing in Park Avenue to the still-visible Times Square signs for theatres advertising "Rent" and "Hairspray." We see Neville driving his car - well, someone's car - across town with his dog, his only companion, trying to kill a deer in competition with a pack of lions; the lions win. But those shots of a deserted city in daylight give us a good sense of how things may be in the days following our destruction of our own race; it's not a pretty sight.

In a series of flashbacks we see Neville saying goodbye to his wife and son as they're helicoptered to safety; he finds a radio station and broadcasts to anyone who might listen that he will be at the South Street Seaport every day at noon if anyone is still listening. These early scenes of a man alone in the world are the best things in the film, along with a brutal scene in which he says goodbye to his dog, who's been bitten by another rabid animal. It's notoriously hard to play on screen to an animal without losing your own focus, but Will Smith does a beautiful job here.

And when we're wondering where the film will go from here, along comes Alice Braga, the Brazilian actress from "City of God," with a boy who's also immune and who heard his radio broadcasts. She's on her way to a community of survivors in Vermont and wants Neville to join her. But he has to work on his vaccine, and may just have found the right one.

Let's leave it there; the problem with "I Am Legend" is that at that point the zombies attack, and that is the weak spot in the film, because the computer generated zombies are exactly like every other zombie film you've seen; faceless wonders with big mouths who move and leap and have strength beyond anything humans can do; at that point "I Am Legend" falls apart.

But up to then, the film is another example of Will Smith's brilliant screen presence; very few actors have that charisma, to hold the screen all alone. By the way, if you're wondering whatever happened to Emma Thompson, she's got an uncredited part as the scientist we see on television at the beginning of the film.