The joke begins like this: A man walks into a talent agent’s office and says that he has a new act, a family act.

“Describe it to me,” says the talent agent reluctantly, not particularly looking to add any new act to his already burgeoning repertoire.

The man goes on to describe some of the most vile, disgusting and repugnant actions you’ve ever heard. These actions may include, but aren’t limited to, incest, bestiality, defecation, domestic violence, abuse, wading in various bodily fluids, and always a spectacular finish, which usually includes a combination of the above.

“My God…” says the agent, clearly disturbed, “What does one call this act?”

The man answers the question with a grand flourish and says, “The Aristocrats!”

This joke, known by comedians as “The Aristocrats,” is a sort of backdoor handshake, a joke that has been told among comedians since the days of vaudeville, as it’s much too “blue” to be performed on stage. As the telling above demonstrates, the joke itself isn’t really all that funny, but it’s the vulgar versions and the coarse improvisations that make it one of the dirtiest and funniest jokes around.

Thus we have “The Aristocrats,” a movie that celebrates comedy and comedians, and one single filthy joke that seems to unite them together. We have all of the big marquee names here: Robin Williams, George Carlin, Whoopi Goldberg and Jon Stewart, all offering their own versions of the joke, or talking about what makes the joke so funny. It is the lesser-known names that really make the documentary as interesting as it is. Bob Saget is enormously filthy here, taking a sharp detour from his family-friendly “Full House” days. Magician Eric Mead tells the joke through playing cards, while a ventriloquist tells a foul version with a dummy. There’s even a mime thrown in.

Yet Gilbert Gottfried provides perhaps the biggest laughs, with an obnoxious tone and sharp delivery that seem to suit themselves very well to the joke. His performance at the Friar’s Club roast of Hugh Hefner shortly after 9/11 shows a comedian that rises to the challenge and succeeds. Co-creators Paul Provenza and Penn Jillette, of the magic duo Penn & Teller, deliver a documentary that shows the uniqueness of comedy and the diversity of the comedian, and it’s the filthiest thing you’ll see this year.