The last man on earth versus a bunch of angry vampires is a killer premise, one that should be easily adaptable in any medium. However, that’s not quite the case, as proved by “I Am Legend.” Originally a novella written by Richard Matheson in 1954, “Legend” has been adapted three times, with director Frances Lawerence and Will Smith at the helm of the third retelling of Matheson’s seminal vampire story.

Matheson’s premise was simple: A mystery virus kills most of humanity and turns the rest into vampires. The lone exception is Robert Neville, an average Joe living in the California suburbs who stubbornly carries on with life. Neville spends his days killing vampires and looking for the scientific causes behind what seems like a supernatural plague.

However, the book has never comfortably made the jump to film. 1964’s “The Last Man on Earth” cast Vincent Price in the Neville role, although, in this case, he was a doctor. This was probably the closest adaptation to Matheson’s book, but the author still disowned it and had his name taken off the screenplay. In 1971, Charlton Heston starred in “The Omega Man,” a rather loose adaptation that substituted crazy albino hippies for vampires.

The newest incarnation of “Legend” is a sort of synthesis of the two. This time, Neville (Will Smith) is a military scientist charged with finding a cure for the virus. He remains immune, but the source of his immunity is unknown. Neville lost his wife and daughter during the social collapse that accompanied the spread of the virus and his only companion is his faithful German shepherd, Sam.

“Legend” starts out solidly, nicely capturing Neville’s fight against the external forces of the monsters and his own internal fight against boredom, loneliness and possible insanity. His days are mundane: a trip to the video store to return a movie, a round of golf on an aircraft carrier in the harbor. In his downtime, Neville continues his quixotic attempt to cure the virus that’s devastated humanity. He experiments on rats and the occasional monster, but nothing seems to work. During a hunting trip in the city, Neville attempts to wrangle himself a new test subject and finds evidence that the monsters may be getting smarter—and more organized. At this point, “Legend” is at its best. The suspense is taut, the danger real. Smith looks as though he really is teetering on the edge of insanity and the complication of smart, deadly mutants seems oh so promising.

Then, “Legend” goes off the rails so spectacularly and quickly that, by the time the credits roll, it feels like you’re walking out of a far different movie than the one you walked into. Things start to go haywire when Neville discovers that he is not the last person on Earth and is forced to interact with Anna (Alice Braga) and Ethan (Charlie Tahan), a mother-and-son pair of survivors. The story of how Anna and Ethan made it to New York is poorly thought out. But, that aside, their sudden appearance seems to be a cue for Smith to change his understated, emotional and relatively convincing performance into the standard Will-Smith-in-big-blockbuster acting. He gives Anna a silly, optimistic speech about the virtues of Bob Marley (which squares oddly with a far more pessimistic “there is no god” tirade he delivers minutes later), knocks off a few funny lines and shoots a few monsters during the climax. It’s almost as though the producers weren’t comfortable enough with Smith on his own and felt a few human co-stars would make things even better. Well, they don’t.

The sudden shift in tone may be just as much the fault of director Frances Lawrence as it is of screenwriters Akiva Goldsman and Mark Protosevich. Lawrence, whose previous credits include the Keanu-Reeves-versus-demons flick “Constantine” and a handful of music videos, never seems sure if he wants to direct a supernatural thriller or a big budget action flick. “Legend” veers wildly between both extremes, but Lawrence never manages to find a way to reconcile the two.

And about the monsters—though CGI effects are more convincing than ever, they’re still not very convincing, and the zombie-vampire-mutants that populate the movie look like they stepped out of an above-average video game. Matheson’s original story is very clear that it’s vampires that are lurking about, but Goldsman and Protosevich’s script never identifies the nature of the beasts. They’re pallid and hairless, with an aversion to sunlight and a penchant for flesh, but they’re also super fast and, judging by the way they bound through abandoned buildings and across deserted city streets, they’re flexible and slightly rubbery, too. When covered in shadows, they’re scary, but in the light, the monsters of “Legend” mostly look foolish.
And, speaking of foolish, there’s the ending, a saccharine bit that has studio mandate written all over it. It’s a final stake in the heart of viewers who, despite the film’s quick collapse, might still be willing to give it a chance. At the very least, when society does collapse and the vampires take over, we’ll have plenty of versions of “I Am Legend” to watch to pass the time.