Somewhat rightfully classified as a B movie, popcorn flick because of its bloat-free length and its casually integrated procession of CGI effects – and these are few of the reasons -, Peter Berg’s “Hancock”, which opened Wednesday in theatres, takes an unconventional and attractive premise then fails to sustain it.

The idea of a superhero that is not concerned about his public image or worries about the aftermath of his actions could have been an diverting comedy, an jittery satire or an inspired combination of both. Instead, a promising beginning gets ruined as the film shifts gears so often it eventually loses focus and impact.

Will Smith plays the self-destructive, anti-hero Hancock in a movie that features a sneaky plot disclosure to put a fresh spin on its storyline. Alcoholic and, of course, hang-over-ed Hancock does not remember much of his past as he goes about interfering in arbitrary crimes and accidents with a disregard that has won him a hand full of supporters around Los Angeles.

That is, until Hancock saves public relations exec Ray Embrey - delightfully played by Jason Bateman - from a speeding train. Ray returns the favor by urging that Hancock enter prison and go into a rehabilitation program before going back to public life as a more reliable, mature citizen and law enforcer.

Charlize Theron spices up the fun as Ray's doting wife Mary, who knows a bit more than her husband about what makes Hancock tick. Being Hancock, Smith is truly funny during the sequences when he is crashing into billboards, exchanging insults with disillusioned spectators, or dealing with the consequences of his actions. Even when his character changes direction from grumpy, sad action figure to an unconvinced, skeptical figure, Smith’s acting remains steady and credible.

The film fails to keep the line it appeared to draw in the largely amusing first half, though, by letting off logic and piling on more chaotically, inappropriate plot turns than it knows what to do with.

What began as a humorous character portrait of a sore superhero soon fades into a fatiguing string of not extremely interesting action set pieces and unfunny, increasingly strained jokes. Moreover, Ngo and Gilligan's script seems to have a crippled exposition that fails to present parts in the story glued fluently one to another.

The script allegedly floated around Hollywood for years, brilliant yet so subversive it was considered all but without the possibility of materialization. Among other difficulties, the movie creators have not bothered to make up any real bad guys for Hancock to fight, and so late in the going it comes up with a stopgap set of bad guys who unleash violence upon Hancock and those he cares about.

And another thing: the PG-13 rating is not at all justified because you will be stunned to discover how violent the picture is.

One sequence involving a bank robbery seems clearly influenced by the famous North Hollywood gunfight of 1997 in which heavily armed criminals with automatic weapons went, in broad daylight, against the LAPD in the city.