"The Aristocrats" is the title of an obscene, provocative joke you've never heard before. That's because it's an in-joke told only among comedians of every age and stripe, going back at least 50 years, when Phyllis Diller first heard it. Comedian Paul Provenza and magician/comedian Penn Jillette have always been fascinated by the joke -- not because of its content but because of the way it's been fashioned by everyone from Gilbert Gottfried to Martin Mull to Steven Wright, who all put their own original spins on the material. The movie investigates the genesis of the joke, the differing ways the comedians approach its telling -- "the singer, not the song" as Jillette puts it -- and, finally, what "The Aristocrats" says about the art and craft of comedy.

It also offers up a smorgasbord of some of today's top comic stars who get to let loose on camera, say things they've never said publicly before and make jokes on subjects about which no one ever cracks wise. Make no mistake, "The Aristocrats" is not for those of tender sensibilities; it's not interested in reaching wide audiences. The film is a timely reminder, instead, that from the days of Lenny Bruce on, the best comedians have always been interested in pushing the envelope. In this case, they go much further, shredding the thing and scattering its pieces to the wind.

Ironically, for a movie created around a joke, the quip in question is hardly a sidesplitter or even that amusing. It's what goes between its set-up and punchline that makes for the laughter. Though it goes on a shade too long, "The Aristocrats" is an intelligent, compelling deconstruction of humor -- a serious examination of the funny bone that's pretty uproarious in its own right. Featuring George Carlin, Chris Rock, Paul Reiser, Robin Williams and Steven Wright. Directed by Paul Provenza and Penn Jillette. Produced by Peter Adam Golden. A Thinkfilm release. Documentary. Not yet rated. Running time: 89 min