There are two momentous performances in the Darwinian horror fable I Am Legend. One is given by the movie’s star, Will Smith, whom I’ll get to in a minute. The other is given by the movie’s practical and digital visual effects — not the ones that bring to life a nocturnal army of shrieking, carnivorous beasties (though those are by no means unimpressive), but rather the ones that render a near-future New York City that has been “ground zero” for a different kind of terror attack — Mother Nature’s. Three years on from a pandemic in which a “miracle” cure for cancer mutated into an incurable, rabieslike plague, the isle of Manhattan has regressed into a state of frontier wilderness, and the images rendered by director Francis Lawrence, cinematographer Andrew Lesnie (who shot the Lord of the Rings trilogy) and visual-effects supervisor Janek Sirrs (a veteran of the Matrix films) have an awesome, iconic power. Driverless cars choke the bridges. Tree roots and tall grass protrude through the surface of Seventh Avenue. And the streets of Times Square bustle with a new sort of tourist — herds of wild deer stampeding through on the run from .?.?. something.

Those somethings are the Infected: human survivors transformed by the virus into ashen, uncomprehending predators who have effectively laid waste to the 1 percent of humanity genetically immune to the plague. By night they take to the streets, unleashing their primordial howls like bats desperate to return to hell. By day, hindered by a vampiric resistance to sunlight, they roost in the shadows, temporarily ceding control of the city to the one remaining noninfected human, the scientist Robert Neville, who has lost his wife and daughter to the virus and who now spends every waking hour searching for a cure. Those, roughly, are the events of Richard Matheson’s 1954 novel I Am Legend, which has been adapted for the screen twice before — first as the Italian-made The Last Man on Earth (1964) with Vincent Price in the lead, and later as The Omega Man (1971), a piece of early ’70s psychedelia that cast Charlton Heston as Neville and turned his adversaries into trench-coated social revolutionaries with a penchant for hep-cat patois.

In Lawrence’s version, which was adapted by screenwriter Mark Protosevich (Poseidon) and revised considerably by Oscar winner Akiva Goldsman (A Beautiful Mind), Will Smith steps into Neville’s shoes, and for the first time an actor has been asked (or allowed) to play the character as something more than God’s lonely, angry man; to show a full range of complex emotions. For much of the movie, it’s literally a one-man show, as Neville goes through his daily routines, tearing about the empty Manhattan streets in his strategically product-placed Mustang Shelby, raiding abandoned apartments for nonperishable supplies, and trapping the occasional Infected so as to have a new trial subject for his laboratory. On the way to “work,” Neville also pops into the neighborhood video store, where he strikes up conversations with the department store mannequins he’s arranged in the aisles and behind the checkout counter. Outside of his trusty German shepherd, these plasticine beings are the closest Neville now comes to meaningful interaction, and Smith plays these scenes with such yearning and tenderness that what might have seemed cornball is instead exceptionally touching (a good deal more so, in fact, than the man-mannequin bonding of Lars and the Real Girl).

Moment for moment, Smith is simply dazzling, and for all the undeniably impressive work the actor has done on his physique for this role, what’s most appealing about Smith is his native intelligence — the way you see him thinking his way through a role — and his capacity for human weakness. Watch him, especially, in the scene where he nurses his wounded canine companion, and later, when he tells fellow disease-free survivor Anna (City of God star Alice Braga) that he refuses to abandon his “post” and follow her to a supposed survivor’s colony in (where else?) Vermont. If he just stays put in his lab, testing one vaccine after another, he’s sure he can put things right, he tells her. There’s a manic edge to Neville by that point — the sense of a rational man confounded by forces beyond himself — and Smith makes you feel every inch of his impotent rage. In what has been a pretty remarkable career up to now, it’s this performance, I think, that fully affirms Smith as one of the great leading men of his generation.

If, as a movie, I Am Legend is less stylistically mind blowing and intellectually ambitious than last year’s Yuletide dystopia, Children of Men, it’s not far off. The screenplay shrewdly condenses the pre-plague back story to staccato flashbacks and manages to shift the emphasis of the novel — which was about how Neville came to be seen as a kind of monster by a new race of nonvampire mutants — without diluting its power. Here, the crux of the narrative is a timely dialectical argument between a man, Neville, who puts his faith in science, and a somewhat fanatical woman, Anna, who puts hers in God. Lawrence’s direction, too, is more subdued and artful than you expect to find in a high-ticket holiday blockbuster, notwithstanding a smattering of cheap shock edits and sound effects used to goose the audience into ?an easy fright. More often, Lawrence (a music video veteran whose only previous feature credit is the Keanu Reeves demon-hunting romp, Constantine) takes things slow and easy, staging much of the film in long, dialogue-free handheld camera shots that use space, production design and intricately layered sound effects to deliver us into Neville’s desolate existence. But when the time comes for the inevitable showdowns between Neville and the Infected, Lawrence is no slouch, including one ingenious standoff in which a winnowing band of daylight is all that separates Neville and his pooch from almost certain doom. If I’ve saved mention of those scenes for last, it’s only because Lawrence, like Peter Jackson and James Cameron, is among the few filmmakers working with full access to the digital paint box who seems to understand how those tools can be used to magnify the human dimension of a movie instead of extinguishing it. He’s like a wizard who turns metal into flesh.

I AM LEGEND | Directed by FRANCIS LAWRENCE | Written by MARK PRO­TOSEVICH and AKIVA GOLDSMAN, based on the novel by RICHARD MATHESON and the screenplay The Omega Man by JOHN WILLIAM COR­RINGTON and JOYCE H. CORRINGTON | Produced by GOLDSMAN, JAMES LASSITER, DAVID HEYMAN and NEAL MORITZ | Released by Warner Bros. | Citywide

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