Stop them if you’ve heard this one ...

A documentary about a legendarily filthy joke, “The Aristocrats” consists of 87 minutes of riffing, joke analysis and reminiscences by a hundred or so well-known comedians, plus dozens of versions of the joke itself.

Described by several comedians as the comedy equivalent of a Miles Davis riff, the joke, like the movie itself, wants to root around in the darkest, most twisted corners of comedians’ minds as they consider the ever-shifting landscape of -- it could be a Time magazine cover, actually -- What’s Shocking Now. Gilbert Gottfried uses it to soothe a prickly audience after telling a premature 9/11 joke at the Hugh Hefner Friars Club Roast. Later, a “South Park” clip incorporates 9/11 into the joke itself.

The joke -- here it is -- is a rambling and entirely improvised porno-scatological ramble sandwiched between a never-changing set-up (“A guy walks into a talent agency and says, ‘I’ve got a great family act for you,’ ”) and punch line (“That’s some act,” says the agent. “What do you call yourselves?” The man snaps a little flourish and says, “The Aristocrats!”). No one can quite agree on what the punch line means, or whether the humor is humanistic (the guy is so clueless he thinks his act is classy); satirical (the upper class can get away with anything); or absurdist. Some tell an inverted version of the joke, in which an absurdly genteel and boring bourgeois family act goes by a disgusting name.

Shot by comedian Paul Provenza and comic-magician Penn Jillette on medium-grade video equipment, the film cuts together interviews with Robin Williams, Drew Carey, Gilbert Gottfried, Sarah Silverman and Paul Reiser, among many others, who extemporize on where they first heard it, what happened to them when they first heard it (Phyllis Diller fainted), what the rules of telling it are, what it means, how it works, and whether it’s still shocking or was ever really funny in the first place. All of these questions are encapsulated in the main reason to see “The Aristocrats,” the aforementioned “South Park” animation that features Cartman telling the joke his grandpa told him, Kyle, Kenny and Stan, while Kyle begs him to stop.

Mainly, though, this “comedian secret handshake” and the most sustained in-joke ever (it’s been around since vaudeville days and has been known to break the 90-minute mark) reveals more about the social dynamic of professional comics than it provides insight into the joke, which is pretty basic. There’s something about professional comedians breaking down what’s funny for civilians that gets annoying after a while. Maybe Provenza and Jillette could have let a little of the hot air out by making them all wear matching T-shirts with a logo like “We Know Funny” or something, I don’t know. Actually, the whole thing reminded me of this joke my grandmother told me once: Two guys make a movie about a bunch of people who pounce on a joke and pick it apart, ignoring its pathetic pleas for mercy, until there’s nothing left of it but a steaming pile of mangled entrails. What do they call it? The Comedians!

*

‘The Aristocrats’

MPAA rating: Not rated

Times guidelines: The movie tells and retells a joke that includes incest, scatology, profanity, bestiality and a talent agent.

A ThinkFilm release. Director Paul Provenza. Producer Peter Adam Golden. Executive producers Paul Provenza and Penn Jillette. Editor Emery Emery. Original score Gary Stockdale. Running time: 87 minutes.

In limited release.