Agent walks into a talent-booker’s office and says, “Have I got an act for you!”

Stop me if you’ve heard this one.

The booker tries to get out of it, but the agent proceeds to “sell” this “family act.”

And what an act. Every vile human excretory or incestuous taboo known to modern humans is detailed.

Dogs are involved. And children.

It starts bad and gets worse.

The agent’s enthusiasm knows no bounds, a full catalog of human depravity.

The booker is revolted but still able to collect himself to ask the punch line.

“So, whaddaya call this act?”

“The Aristocrats!”

That filthy joke, in all its uncensored and not particularly funny glory, is the starting point of “The Aristocrats,” a hilariously raunchy and revealing documentary about a famous joke that comedians like to tell each other. That’s how it begins, but what the film really reveals is the role of the comedian in society and the craftsmanship of joke-writing and joke delivery.

Whoopie Goldberg

It’s dirty, sure. If you’re the sort of person offended by language, stay away.

But what’s fascinating is hearing generations of comics – from Shelley Berman and Phyllis Diller to the Smothers Brothers, Robin Williams to Rita Rudner, Carrot Top to Trey Parker (“South Park”) – tell the joke and explain the joke, how it should work or could work better.

The movie’s tag line is “All you have to remember is one word” (Aristocrats) to tell it. It’s all about improvisation, what you do to shock, offend or ironically mock to get to that word.

Not everybody can manage it. Drew Carrey scores; Steven Wright, Richard Jeni and Kevin Nealon bomb. Larry Storch of TV’s “F-Troop” does it in a British accent and makes it hilarious. Phyllis Diller faints. Eric Idle doesn’t get it.

He’s not alone. By itself, this joke, which goes back decades (Chevy Chase used to hold parties/contests to see who could stretch it out the longest), is just filthy and appalling, the lowest of low comedy. But it’s a great crucible for the comic art.

Paul Reiser explains it and works it. Richard Lewis hysterically condemns it. George Carlin breaks it down. Robin Williams riffs around it. Eddie Izzard tries to fix it. Bob Saget blows his “Full House” wholesome image to bits in one long, profane and cathartic telling.

Andy Dick.

What becomes clear as we are numbed to the joke itself is that these people are part of that comic continuum, a noble profession of fools, mocking society’s taboos even as they realize that there’s some stuff you dare not do on stage. Yet.

Comic magicians and cultural gadflies Penn and Teller ably play host to the film; comic Paul Provenza directed. Like Jerry Seinfeld’s “Comedian,” it’s a movie that pulls back the curtain on the world of comedy – the who, how and why of funny people doing funny stuff.

Gilbert Godfried

It shocks. But the biggest shock isn’t the dirty, dirty joke or its dirtier variations. It’s the comic whose timing, delivery and sense of when the right moment was to make it public places him on a pedestal that leaves his peers in awe. You will be amazed.

It’s funny. It’s filthy. And as a documentary about the comic art, “The Aristocrats” is smarter than it lets on.

But don’t try this at home. These people are professionals. A civilian could have her mouth washed out with soap for talking like this.