He's at it again. After punching out a tentacled alien and dragging it across the desert, being splattered by the guts of a giant extraterrestrial cockroach, taking on some disgruntled robots, and battling a group of even more pissed off vampire zombies as the last man on earth, Will Smith is saving the world one more time.

Donning a superhero outfit (if you can call a skanky beanie, old T-shirt and greasy jeans a superhero outfit), the actor must now fight inner-city crime, a hostile public, hangovers, and the ultimate nemesis: indecisive filmmakers. 'Hancock' begins as a breakneck comedy, with Smith flying high (and comfortably) as the bum whose superpowers are eclipsed only by his bad attitude and babbelas breath. A reluctant saviour at best, his destructive rescue missions and drunken exploits have become Youtube fodder and the bane of city officials tired of footing the bill. But he doesn't give a damn. So it takes a lot of convincing from idealistic PR consultant Ray Embrey (the always funny Jason Bateman), and a few more expensive mishaps, before Hancock agrees to an image makeover.

But just as the hobo transforms into a professional crime fighter with his own 'catchphrase' ("Good job"), so does the film. The hugely entertaining, borrowed-from-'The Incredibles' premise is suddenly replaced by a more traditional, earnest comic book concept: the hero grappling with his inner demons and outsider status; the quest for love and acceptance; a threat to the invincibility; and the beckoning of natural phenomena for a near-apocalyptic showdown.

The twist may be surprising, but the second half of the film just doesn't work. Treating humour like Kryptonite, it's poorly conceived and executed, simply using visual effects to brush over inconsistencies in the rudimentary plot. And with director Peter Berg losing grip of his tight little 90-minute film, even actors like Smith, Bateman and Charlize Theron (as Embrey's wife) and an entire subplot involving a prison break, are completely blown away by the CGI thunderstorms.

'Hancock' will no doubt cement Smith's status as Hollywood's real-life superhero and become his eighth successive $100-million-grossing film, but it's by no means invincible.