Just to get a sense for how long Mark Protosevich’s script for I Am Legend has been bumping around Warner Bros.: Bill Clinton was still president. The lead role was going to be played by the governor of California, a guy who used to be a movie star named Arnold Schwarzenegger. And a $100 million budget was considered crazy enough to shut down production.

Scripts don’t hang around like that because they stink; bad scripts in Hollywood either disappear entirely, or quickly become Wayans brothers comedies. No, I Am Legend was a challenge because it was going to demand something from its actor, and even from its audience. Studios make post-apocalyptic thrillers because they expect them to be exciting. They do not expect a portrait of existential crisis.

But that’s really the heart of this third cinematic adaptation of Richard Matheson’s 1954 novella. In 2012 Manhattan, Robert Neville (Will Smith) appears to be the only survivor of a virus—originally a genetically-engineered cure for cancer—that wiped out most of the human race three years earlier. Others reacted differently to the virus, becoming mindless vampire-like creatures that emerge only at night, forcing Neville to hole himself up in his Central Park townhouse. Fortunately for Neville—and fairly preposterously for the audience—he’s not just a military man who knows his way around weaponry, but a virologist who can spend time working on a cure based on his own immunity in his fully-equipped basement lab.

Neville’s multi-disciplinary awesomeness—a far cry from the Everyman of Matheson’s novel—is indeed pretty lame, but it pays to get past the changes to the protagonist. Director Francis Lawrence (Constantine) effectively employs digitally-doctored images of Times Square, the Brooklyn Bridge and other New York landmarks to capture the bustling city rendered shockingly silent, but the impact comes from more than the imagery. With no one around but his faithful German shepherd for Neville to exchange witticisms with, the film as a whole stays fairly quiet, including spare use of James Newton Howard’s score. Some of his set pieces—including the accidental discovery of a “nest” of the infected, and infected dogs waiting for night to fall—are some of the finest, most tensely crafted scenes you’ll find this year. And no one has to shout at you to let you know how thrilling it all is.

Just as thrilling, in a completely different way, is the central character. He may be Machine-Gun-Toting Bio-Savior Man, but Robert Neville is also a man, and a desperately lonely one. Lawrence captures him not just prowling the city for the wild deer that roam the streets, but prowling a video store he has populated with mannequins. There aren’t many actors like Smith who can carry a quiet drama like The Pursuit of Happyness and a futuristic blockbuster like I, Robot, but he combines those two personas here for an involving portrait of a guy trying to convince himself there’s a reason to stay alive. His interaction with his dog improbably becomes one of the most touching screen relationships of the year; his word-for-word accompaniment of Shrek hilariously but vividly captures someone whose only companionship has been pre-recorded. Stretch your imagination to picture Ahnuld pulling that off.

I Am Legend has so much going for it for so long that it’s a shame how much it falls apart in its third act. A swarming vampire attack provides the obligatory big finale, and the story begins to hint at an optimism utterly absent from Matheson’s source material. The film ultimately becomes something you’d expect from a focus-group-approved blockbuster, but that shouldn’t negate the surprising impact of an action-drama that takes psychological realism seriously. While it may have taken a long time to wind its way to the screen, I Am Legend does deliver more than a tale of the living dead. It’s about why the living shouldn’t want to join them.

I AM LEGEND

*** (three out of four stars)

Starring: Will Smith

Directed by Francis Lawrence