Whenever there's a problem that threatens the entire planet, Will Smith is on the scene. Whether it's rumbling with invading extraterrestrials (Independence Day), keeping neighborhood aliens in line (Men in Black), or corralling anthropomorphic robots who just don't want to get along (I Robot), Smith is your ultimate white hat. Now, in I Am Legend, he's back in action to save humankind from itself.

Problem is, he's too late. As the film opens, we find Smith as the only bit of humanity left in Manhattan. He alone can enjoy the Big Apple's urban amenities and stiff sales tax. Smith strongly suspects he's the only normal person on Earth (a belief he shares with many of my relatives).

What happened? Well, it's not space aliens that are making sure no one has a nice day, nor is it androids lacking in social skills. It's not even a trendy environmental catastrophe invoked by Hollywood scriptwriters to wipe the world clean of Homo sapiens. It's our own scientists, once again doing more harm than good.

The fear of genetically-modified food has been slightly transmogrified into a horror tale of genetically-modified medicine. Imagine this: it's but a few years hence, and science has conquered cancer. Yep, just one little pill, and you can stop worrying about America's Number Two Killer. The trouble is you have to worry about Number One - which is that little pill. Despite tests on ten thousand people, the nerds down at your local research lab have pulled a boo-boo. It turns out there are some less-than-desirable, long-term side effects to this wonder drug, most notably death. Ninety percent of the population takes the pill, and succumbs pronto. Most of the rest get turned into a cross between a vampire and a zombie. That's an unattractive hybrid incidentally, which warps their personality to that of a rabid dog, and adds insult to injury by morphing their complexion to that of the Borg Queen.

Well, with six billion dead patients, you might cautiously venture that this biotech cure is worse than the disease, and the inventor is probably not going to get the Nobel Prize for Medicine this year.

In fact, no matter what you think of the Food and Drug Administration's lack of speed in approving new therapies, you'll come away with a finer appreciation for their conservative tendencies after seeing this flick. Remember Thalidomide? Europeans OK'ed that "wonder drug" too quickly, with horrific results. Well, this time the Americans have been too soon seduced by a miracle cure, and - somehow - Big Pharma has managed to make sure that every man, woman, and child on the planet gets their dose of the engineered virus right away (something that hasn't happened with AIDS medication, or even aspirin.) In reality, of course, you would think that a cure for cancer would be a pretty expensive and difficult-to-obtain drug in places like, oh, say rural Bangladesh. At least until the patent ran out.

OK, so that's a little wonky, but most of the medico stuff in this film is moderately credible (except, that is, for the basic premise) - due, no doubt, to the twenty or so doctors you'll see thanked as consultants in the endless credits. Still, the mutant "zompires" are surprisingly malevolent powerhouses, and - contrary to any life form you're likely to meet in the visible universe - they have absolutely zero tolerance for ultraviolet radiation. Even a momentary exposure to a shaft of afternoon sun is lethal. But rather than slathering on Number 500 sun block, or even just putting on more clothes, the mutants prefer to stay huddled up in hidden, indoor hives during the day, coming out at night to hunt down Smith, whom they see as nourishment. Come to think of it, that's a bit weird, too. Plenty of wild animals cavort through the city - escapees from the zoos and the parks - but the mutants have gone completely cannibal, and ignore all this feastible beast. Maybe they'll consider a switch to other meats once Smith has been turned to zompire chow.

I Am Legend is, principally, situational. Sure, Smith (a virologist who's set up a basement lab to figure out how to beat this bad bug) occasionally busies himself with his research, and now and then takes on the zompires. But mainly what he does - and what you'll find most memorable - is enjoy New York gone to seed. It's the Big Apple as you know it (after all, it's only 2012), except it's been without maintenance for three years. The pushy residents are gone, too. Imagine the possibilities. Imagine the lack of traffic.

And, if you can't do that, just sit back and feast your eyeballs on the oddly pleasant sight of Times Square overgrown with weeds. It's The Day After meets Escape from New York.

Well, who knows? Maybe engineered Armageddon is in the cards. After all, doesn't the Mayan Calendar predict the end of the world in 2012? Perhaps those long-vanished natives had some uncanny foresight about medical advances of the 21st century.