He was a cocky American gangster, an ice-blooded CEO of the heroin trade who lorded it over Harlem in the 1970s. He ruled by violence, murdering anyone who dared to cross him. For years, he defied the cops. There was one key quality, though, that he didn’t share with Frank Lucas, the stoic underworld antihero of American Gangster: Whatever law he broke, he had a down-and-dirty good time doing it. I’m talking about Nicky Barnes, Lucas’ chief rival. The mesmerizing documentary Mr. Untouchable, produced by Damon Dash and directed by Marc Levin, traces Barnes’ astonishing rise from junkie to godfather. It’s a movie that brims with tales of treachery, of nasty pleasure, of a depraved hustler king who surrounded himself with soldiers he called the Council, whom he protected until he was betrayed by them. At that point, he committed the ultimate gangster no-no (ratting out your friends), which he turned into the ultimate street vengeance. In American Gangster, Cuba Gooding Jr., in mack-daddy hats and furs, plays Barnes as a charismatic clown, but Levin found and interviewed the real Barnes, who is still under the Witness Protection Program; even in the shadows, he exudes the magnetism of rationalized brutality. To watch Mr. Untouchable is to get deep beneath the surface of the thug life American Gangster only scratched.