With great power comes great responsibility... not that bona fide superhero Hancock (Smith) cares.

The eponymous protagonist of Peter Berg's all-action adventure is another victim of our celebrity-obsessed times, turning to the whisky bottle to cope with the pressures of fame.

Unfortunately, this self-loathing crime fighter has no time for rehab - all he wants is a park bench to rest his weary head, from where he can spout obscenities at passing women and children.

For the opening hour, Hancock delivers everything you expect from a Will Smith blockbuster including spectacular set pieces and wisecracks, albeit with an excess of strong language and crude humour, which should concern parents of younger children.

The piece de resistance is the superhero serving time in prison and exerting his dominance over two thuggish inmates by threatening to shove one man's head where the other's sun don't shine. Eye-watering facial expressions and nifty special effects prove Hancock is true to his word.

This time behind bars sows the seeds of the character's redemption and then, without warning, Berg's film abruptly morphs into something else entirely.

The mood turns increasingly serious as scriptwriters Vy Vincent Ngo and Vince Gilligan throw us a succession of narrative twists, hastily introducing a pivotal character to explain Hancock's back story and propel the film to its sombre resolution.

As the one and only saviour of Los Angeles, Hancock begrudgingly rescues beached whales, thwarts bank robbers and helps to maintain peace on the city streets, except his headline-grabbing exploits always result in millions of dollars of damage.

Drunk, foul-mouthed and deaf to an ever-growing chorus of public dissatisfaction, Hancock rescues publicist Ray Embrey (Bateman) from a collision on a level crossing. As a thank you, Ray - the 'Bono of PR' - vows to restore faith in the misunderstood superhero.

Hancock potentially offers great value for money: two screenplays - one a comedy, the other a doomed romance - for the price of one ticket.

Evidently, writers Ngo and Gilligan hope to please everybody, some of the time. Despite the haphazard plotting, the second half is much more interesting - but hinges on a twist that can't be revealed or discussed here.

Smith's natural likeability smoothes some of his character's rough edges, but he delivers his best work when Berg's film cuts the comical asides and forces the superhero to take a long, hard look at himself in the mirror.

Bateman and Theron are underused and director Berg orchestrates the action sequences with aplomb, ultimately letting the digital trickery run rampant.