In “Hancock,” Will Smith is to superheroes what Billy Bob Thornton is to the department store Santa Claus in “Bad Santa.”

He’s faster than a speeding bullet, more powerful than a locomotive — well, you get the picture. He’s also an unapologetic booze hound, he dresses like a panhandler and he has an inventive potty mouth. He’d make a good date for Amy Winehouse.

But they’re not Hancock’s biggest problems. He’s a committed crime fighter and guardian of the public safety but he’s also an irresponsible good guy who reels into highway signs, crashes into buildings and ratchets up millions in damages as he goes about his business.

Cut to mild-mannered PR guy Ray Embrey, played by Jason Bateman. Ray is an idealist who wants to use his promotional skills to better mankind. When he pitches an idea to the execs of a big drug company, he’s practically hooted out of the board room. Ray proposes that they give away new TB-fighting medications in order to be eligible for public recognition of their humanitarianism. Free is a four-letter word to the honchos and they can’t spell humanitarianism.

And that’s not the nadir of Ray’s day. On the way home, he’s trapped on railroad tracks with an oncoming train headed his way in a hurry. Hancock drops from the sky, wrecks the train and saves Ray’s life. However, an adoring public is not on hand.

He’s castigated for his heavy-handed rescue, which gives Ray an idea. He decides to revamp Hancock’s image. At this point, the movie continues to create a giddy sense of comic anarchy that reworks the superhero genre. At the same time, it underscores the importance of image in our culture.

Ray brings Hancock home to meet his family and a little mystery ensues. Wife Mary, played by Charlize Theron, exhibits an unaccountable animosity toward Hancock. There’s some history here, we presume, and some chemistry.

Hancock agrees to follow Ray’s plans and goes to prison for some of the crimes he’s racked up while fighting crime. When he gets out, he’s clean shaven and he agrees to wear a superhero suit that he finds ridiculous.

Subsequently, the new and improved Hancock becomes less interesting and so does the rest of the movie. Directed by Peter Berg, the movie succumbs to script problems that attempt to explain Hancock’s past. Because of amnesia, he himself has no knowledge of that past yet it obviously is the cause of his angst and disillusion.

Most superhero movies call on a mythology to explain the origin and extraordinary powers of their characters and they range from scientific experiments gone awry to visitations from other planets. They’re gimmicks, though generally effective gimmicks, but there’s nothing that concrete here.

As the attempts to explain the nature of Hancock become fuzzier, the tone becomes more somber and sentimental and violent.

Still, Will Smith is undeniably cool, Theron undeniably watchable and Bateman undeniably decent. “Hancock” doesn’t live up to its hip and irreverent premise but it doesn’t exactly crash and burn either.