The most awesome power of Will Smith’s comic-bookish dramedy Hancock is its ability to turn superhero clichés upside down. As John Hancock, Smith demonstrates talents comparable to Superman’s: incredible strength, flight, invulnerability, etc. Hancock has a similar status to the last son of Krypton, being apparently the only super powered being of his kind on Earth. Both characters strive to use their godlike abilities to help mankind, but there the resemblance ends.

For all of Hancock’s impulses to foil crime and prevent accidents in Los Angeles, at some point before the film begins, he seriously let himself go. A grumpy, accident-prone boozer, Hancock serves as a role model for none, and his butter-fingered rescues invariably cause more trouble than they relieve.

Directed by Peter Berg, Hancock’s revisionist take on superheroes at first exudes confidence, offering an amusing metaphor for celebrity meltdowns and providing a reality check for the comic-book fantasies that dominate summer movies. But where most caped crusaders have secret identities, the film suffers from a full-blown identity crisis. After its first hour, Hancock loses track of the kind of movie it wants to be and falls all over itself.

At first, Hancock seems capable of coasting on its elaborate, funny CGI sight gags. Without a catchphrase like “Up, up and away,” Hancock’s signature may be the way he comes in to land too fast, invariably touching down with a spray of shattered pavement. “It was like that when I got here,” he mumbles after one barely controlled crash.

At one point, Hancock saves Ray (Jason Bateman) from a car trapped at a railway crossing, but unwittingly sets off a chain reaction of train cars rear-ending each other.

Ray shows his gratitude by inviting the despised superhuman over for dinner with his hero-worshipping son (Jae Head) and strangely hostile second wife Mary (Charlize Theron). A public relations expert struggling to get a philanthropic venture off the ground, Ray takes on Hancock as kind of a pro bono project. Ray informs Hancock that he has an “image problem,” but the joke is that the public perception of him is completely accurate.

It’s hard to imagine any actor playing Hancock as well as Smith. At some point– I suspect some time between ID4 and Men in Black– Smith transformed from hyper hip-hop artist and TV comedian to a charismatic movie star. Maybe he was bitten by a radioactive movie camera. In Hancock he takes advantage of his rangy frame for physical comedy, whether sprawling face down on a prison cot or hurtling through the air, arms and legs waving spastically. Smith humanizes the role with subtler touches. His slower, more growly delivery conveys the character’s disdain for the mere mortals who surround him. His eyes, however, contain a wounded quality that sells the notion of his secret loneliness.

This summer’s big superhero movies build to showdowns in which the main character battles some kind of huge CGI adversary. Hancock’s best set piece generates suspense not over whether he can thwart an Iron Monger or an Abomination, but whether he can save the day without harming innocent bystanders or otherwise fucking up. In a way, the sequence puts heroism in a realistic context: In real life, we face the challenge of “not fucking up” far more often than “defeating hulking supervillain.”

Hancock’s first hour dispenses with the standard-issue origin story, and does just fine without it. Superheroes are so familiar in pop culture that we can grasp the film’s conflict the first time we see Hancock flying erratically, looking like he slept in his clothes. Bitter, drunk superhero trying to turn his life around? Got it. Love it.

Then Hancock loses its faith in its premise and labors to concoct an explanation. Berg and the screenwriters offer a major, game changing plot revelation, and then bombard the audience with new rules but no time to process them. In essence, the story switches from Hancock’s internal struggle to a specific, stormy relationship, and thus fails to follow through on the film’s best ideas. The film’s last act sadly emulates the stumblebum hero of the first, and Hancock, despite great power, fails to save the day.

HANCOCK ( 2½ ) Directed by Peter Berg. • Starring Will Smith, Jason Bateman and Charlize Theron. • PG-13, 92 min. • At the Century Cinemas Del Monte Center, Lighthouse Cinemas, Maya Cinemas, Northridge Cinemas.