Hancock is something of a one-trick pony. It attempts additional tricks in its woefully miscalculated second half, but they all misfire. Which leaves us with the one.

Our hero, you see, is a superman. But he is also a deeply flawed and troubled man. At the start of the movie, we find him in an alley, homeless, disheveled and hung over. A child attempts to wake him and direct his attention to a life-or-death situation unfolding that very minute on a Los Angeles freeway. The man of steel is such a mess that the kid walks away shaking his head in disgust.

We quickly come to see why the citizens of this metropolis have mixed feelings about their guardian. As played by Will Smith, he’s the kind of superhero who rockets into the sky and takes out the bad guys. He’s also the kind who’s usually had so much to drink beforehand that he nicks chunks out of tall buildings, knocks down traffic signs, damages landmarks and tears up streets with his unsteady landings.

Smith’s character and the populace he protects have a love-hate relationship. The people need him. They just don’t need the millions of dollars’ worth of collateral damage his buzzed crime-stopping tends to produce. For his part, Hancock appears to be troubled by two things: the lack of gratitude for his good works and the fact that he’s the only one of his kind on Earth.

Popular culture is currently overdosing on superheroes, so it was only a matter of time before a cheeky writing team (Vince Gilligan and Vy Vincent Ngo) decided the moment was ripe for turning conventions on their heads and gave us a super anti-hero. Consider for a second the ever-changing line-up of actors playing Superman, Batman and Spider-Man. (Rumor has it Tobey Maguire has opted out of Spidey 4.) Then we have the multiple Hulks, the X-Men, Daredevil, Elektra, the Silver Surfer and Iron Man movies, even Wanted and on TV, of course, “Heroes” features a new supergeneration, the first not descended from a comic book. Is it any surprise we’ve reached the tipping point where these super-folk are concerned?

No. The only surprise is that director Peter (The Kingdom) Berg and company don’t wring more from the picture’s groundbreaking premise. The story begins promisingly enough. A drunken Smith wreaks comical crime-fighting havoc, which is convincingly computer-generated. He also saves the life of a likable PR guy, played by the always-likable Jason Bateman. Ray is determined to repay his debt by refurbishing the derelict crusader’s image. To that end, Hancock must repay his debt to society by serving a jail term for destruction of public property and receiving counseling for his problems with anger and alcohol. Ray’s instincts tell him the crime rate will rise in the interim, and the people of L.A. will come to realize how much they really need Hancock.

The first half of the film is periodically amusing, particularly the scenes where Bateman’s marketing whiz offers Smith tips on increasing his lovability quotient: Tell police they’re doing a good job; ask politely for permission before pulling someone from a perilous situation; land more carefully. People are happier to see you when you don’t destroy their streets, he observes. When the time arrives for Smith to put these lessons in practice and to don the form-fitting outfit Bateman’s had made for him the movie hits its humor high point.

And then, without warning, Hancock morphs into a completely different movie, one that doesn’t make sense and doesn’t make us laugh. The script switches gears with possibly the dumbest plot twist in motion-picture history, and the change in tone saps the proceedings of fun as swiftly as kryptonite does Superman of strength. Ray’s wife Mary (Charlize Theron), previously an incidental player, figures prominently in the movie’s second half and let us just say this is not the Oscar winner’s finest hour on the silver screen. I don’t believe we’ve seen work from her on a comparable artistic level since the days of Mighty Joe Young.

Movie-critic law forbids me saying more about the big surprise, except that it essentially divides Hancock into two distinct films. The first is a semi-clever romp that benefits from fine effects and the chemistry between Smith and Bateman. The second is an overblown bit of button-pushing in which the filmmakers grope for emotional, spiritual and metaphysical pay-offs they haven’t remotely earned. It’s a train wreck. Too bad somebody from Sony Pictures didn’t come soaring to the rescue.