There are moments of serene, otherworldly beauty in I Am Legend, a film as much about loneliness as vampire zombies attacking the Fresh Prince of Bel Air. Based on Richard Matheson's 1954 novel (which also inspired Charlton Heston's pulpy The Omega Man in 1971), I Am Legend takes place in a world largely absent of humanity—a world in which one man spends his days as he pleases and his nights fearing for his life.

Actually, in this I Am Legend, the focus isn't on an empty world so much as it's on an empty New York: In director Francis Lawrence's post-apocalypse, water has flooded the Lincoln Tunnel, Manhattan's skyscrapers stand hollow, and weeds raggedly sprout from cracked pavement. Billboards advertise to no one, rusting cars litter 42nd Street, and—aside from a whisper of birds' wings, or a hushed breath of wind—all is silent. When Robert Neville (Will Smith) stalks a deer through the ruins of Times Square, both he and the deer have ample places to hide among tall, thick grass.

There's an ethereal loveliness about it all, an almost exquisite sense of melancholy. Until, that is, the sun sets, when the air becomes filled with inhuman screams. Thanks to what I Am Legend calls a "viral cure for cancer" that pretty obviously went awry, most of Earth's population has become nocturnal monsters who scream like banshees, feast on whatever flesh they can find, and look like hybrids of vampires, zombies, burn victims, and that one albino dude from The Princess Bride. Needless to say, the Fresh Prince isn't gonna let that shit stand.

For its first two-thirds, I Am Legend captivates with its steady, dark patience: Music video director Lawrence, who also helmed 2005's Constantine, lets his striking visuals and Smith's considerable charm carry the film. The details, perhaps, are better than the whole: Neville—who's not only the last man on Earth, but also, natch, a former military scientist—desperately searches for a way to cure the "elegant" virus that's wiped out humanity, but he also staves off madness by watching recorded TV, having stilted conversations with mannequins, and slowly working his way through every DVD at his neighborhood video store.

I Am Legend is at its best when it's most poetic or frightening: An unabashed horror flick, the film's strongest moments, aside from Lawrence's painterly shots of a decomposing New York, are Neville's genuinely frightening encounters with hives of vampires. Despite making the confusing decision to render most of the film's villains in CG, Lawrence demonstrates a great eye and an excellent sense of tension.

And then it all kind of goes to shit. There are a couple of truly silly moments in the first chunk of I Am Legend (bewilderingly, all have to do with Bob Marley), but it's not until the real plot kicks in that things get iffy, as screenwriters Mark Protosevich and Akiva Goldsman start cramming in unbelievable fellow survivors and sloppy deus ex machinas. In the film's third act, the pragmatic fear that defines the rest of the film is surrendered to brash, weak sentiment; soon, flaws that were previously easy to overlook, like the rubbery CG bad guys, begin to grate. Early on, in one of Neville's harrowing flashbacks, Lawrence has military jets blow up Manhattan's bridges in a brutal attempt at quarantine. It's an astonishingly surreal and cinematic image, but it also holds an unsettling core of uneasy emotion. Instead of holding onto that unique tone of sadness and terror, I Am Legend's simple, sharp premise grows continually more cartoony and dull. Not to advocate zombie vampire plagues or anything, but maybe Neville didn't know how good he had it when he really was the last man on Earth.