Picture a professional boxer punching you in the head. Pretend you survive. Now picture the same guy continuing to punch you in the head for the next 20 years.

From 1985, when he won his first pro bout with a first-round knockout, to 2005, when he suddenly quit the fight business midway through a humiliating beat-down by opponent Kevin McBride, Mike Tyson absorbed that sort of punishment on a fairly regular basis. You can see the result in "Tyson," the stunning new documentary by director James Toback. The former heavyweight champ, now 42, fills the screen, adrift in amiable confusion ("I met the president of Chechnya, I met the president of Istanbul") and veering off into private worlds where he hears voices he can't quiet. Now, looking back over his outlandish career, he says, "Everything was totally cryptic to me."

What makes the picture so enthralling are the moments when Tyson's interior fog clears and he enters a state of unexpected clarity and insight, revealing a rough poetic soul. ("Money is like paper blood," he says, in his surprisingly high-pitched, lisping voice. "You need it to live.") As Toback observed after an early screening of this film a few months back, "He has these different selves, in a sort of Whitmanesque sense — he contains multitudes."

They're all on display here, sometimes all at once; because Toback, working with the editor Aaron Yanes, has created a new documentary form. Apart from the requisite archival fight footage of "Iron Mike" destroying one contender after another, the picture is constructed entirely from 30 hours of interviews the director conducted with Tyson over the course of a week and then cooked down, over the course of a year, into a 90-minute film. Toback turns this static footage into a thrilling experience, though, by crowding the screen with multiple panels of interview segments, weaving sound and image together to create a sense of Tyson talking among his selves. It's a spectacular effect.

The ex-champ has done things in his life that most people could probably never forgive — chief among them the rape of a black 18-year-old beauty queen in an Indianapolis hotel room in 1991, for which Tyson served three years in prison. He's still furious about what he insists was a wrongful conviction in that case: "I may have took advantage of a woman before," he says bitterly, "but I never took advantage of her." On the other hand, he's candid about his inclinations. "I want a strong woman," he says at one point, "and then I want to dominate them sexually."

An appalling childhood in the worst precincts of Brooklyn provides some context for his youthful brutality. A fat kid with asthma and glasses (and that lisp), he began getting into fights early. There were many arrests, and at the age of 12, he took his first trip to a juvenile correctional facility, where he encountered some familiar neighborhood faces. "It was like a class reunion," he says. "I started going there on a often basis." It was in juvie that his talent for boxing was noticed, and his life began to take on meaning. At 14, he competed at the Junior Olympics, where he knocked out another kid in eight seconds. In 1986, at the age of 20, he became the youngest heavyweight champion on record. Life got good, but then bad, and then much worse.

At his peak, Tyson was world famous, and blindingly successful. He didn't earn millions of dollars; he earned hundreds of millions. But he got sloppy. He signed on with the notorious promoter Don King. ("So many leeches," he says, "that was my downfall.") He got lax about training, and started losing fights. There was a harrowingly ugly divorce from TV sitcom star Robin Givens ("I felt like half of a person"); and the famous ear-biting match with Evander Holyfield, which cost him his boxing license. And then the prison stretch, too.

In "Tyson," we meet a man who's fallen from the heights down into a hole. The money's all gone now ($300 million is the estimate — the champ didn't keep meticulous records), and many people still see him as little more than an animal. But Toback, a longtime friend (he even cast Tyson in his 1999 movie "Black and White"), enlarges the frame to show us more of the picture. And we see that after all the years of rage and calamity, Iron Mike appears to have found some kind of lonely peace. Viewing the completed film for the first time, he told Toback, "It's like a Greek tragedy, except I'm the subject."

Don't miss Kurt Loder's review of "The Informers," also new in theaters this week.

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Check out everything we've got on "Tyson."

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