A startling documentary about stand-up comedy, which has been a surprise box-office hit in America, is released here next week. Made by the comedians Paul Provenza and Penn Jillette, it features clips of more than 100 performers, including Whoopi Goldberg, Robin Williams, Carrie Fisher, Billy Connolly and Eddie Izzard, telling variations on a joke known as The Aristocrats, which concerns an obscene vaudeville act.

The film becomes a hilarious and often moving treatise on shock, surprise, taste, humour, the art of storytelling and the creative imagination. After an Edinburgh Film Festival screening late last month, enthusiasm for The Aristocrats spread like Asian bird flu through the comedy community on the Fringe. We may already be living in the post-Aristocrats era.

Apparently, the Aristocrats gag, though never repeated on stage, has been a dressing-room staple of American comics for decades. It begins with a man entering a showbiz agent’s office to pitch him a nightclub act comprising a man, his wife and their two children, whose performance is then described in as much pungent, pornographic and scatological detail as possible, limited only by the imagination and scruples of the teller. The horrified agent then asks what the act is called. The man replies: “The Aristocrats.”

The humour arises from the contrast between the repellent nature of the act and the polite, aspirational title it has been given. The lure of the gag, for the film-makers, was the infinitely extendable central section, which pushes boundaries of both endurance and taste, and can be stretched out for as much as an hour and a half.

“The editing process reflected a conscious decision to make the movie about ideas,” Provenza explains. “In fact, we ended up doing some comics a disservice, because we didn’t necessarily use their funniest bit, but the bit that best helped to illustrate the ideas of the movie. It starts repetitively, but if you listen to each version of the joke, hearing the same gag again and again shows how people take off with it and create different things. In the first six minutes, George Carlin does a version that’s totally grossed-out and scatological, then we move on to Drew Carey teaching us how to do the joke, then into a riff constructed entirely of little soundbites, until, I think, your moral judgment is suspended. You’ve been bludgeoned. Boom! Then you can concentrate on the absurdity of the thing, the structure of the gag and the different layers of offence. It’s about the singer, not the song. Repeating the same joke allows us to get over the issue of content and concentrate on the thorny issue of aesthetics.”

Somewhere around the midpoint of the film, after an especially hilarious sequence in which a clown-faced mime acts out the gag silently on Venice Beach, there’s a 20-minute section when, for me, the joke wears thin. I began to feel as if I was being dragged through a trench of filth, and the violence against women in the various versions of the story became so relentless that when Bob Saget described one of the male performers smashing his penis repeatedly into a drawer, I was almost relieved. That said, other sections of the comedian-packed Edinburgh screening were still howling. Seen in public, The Aristocrats becomes a living object lesson in the fact that a one-size-fits-all approach to making decisions about what is acceptable just won’t fit. It doesn’t even work in one room full of people who all do the same job.

In its closing section, The Aristocrats transcends its base subject material to become genuinely profound and emotional. We are softened up for the final sequence with a specially made South Park short in which the animated toddlers describe a version of the vaudeville act where the perverted family run around impersonating the victims of the 9/11 disaster while covered in various bodily fluids. Next, we go to a charity event filmed in New York three weeks after 9/11 itself: “The shock of hearing the South Park bit takes us close to the state of the room when Gilbert Gottfried takes the stage at a Friars Club roast,” Provenza says of the startling closing set by the infamous American humorist. “Inadvertently, we had somehow created our own third act of The Aristocrats. We had shot Gilbert doing the joke in private three or four weeks before 9/11, so it was in his mind.”

On stage, Gottfried’s gag about taking an internal flight with a connection at the Empire State Building dies. Someone shouts: “Too soon.” “You can see him stall,” Provenza remembers. “His fingers twitch, then he decides to start the Aristocrats gag. He didn’t plan to do it. But if you look at his face, you can see him doing the math in the moment. He chose it for a reason. The room was full of comedy pros busting his ass for ‘crossing the line’. And all around town, the comedy clubs were closed, and club owners were asking when it would be time for people to start laughing again. Gilbert was proving a point. The gag became a kind of safety rope. It was all about crossing the line. And he knew an audience of comedians would intuit the subtext. He was asking us when it’s okay to laugh. The transgressive nature of the piece was the cathartic relief that everyone wanted after the confusion of 9/11.”

Cutting between Gottfried’s grinning face and the sight of people literally falling off their chairs laughing, and gasping in pain for breath, The Aristocrats makes a convincing case for absurdity as a logical response to tragedy. I wept, not tears of laughter, but tears of joy. I wept tears of joy watching a tiny man describe a family of four sexually and physically abuse each other, and any animals in the vicinity, in the name of entertainment. And after an hour and twenty minutes of The Aristocrats’ surgically precise analysis of how we are made to laugh, I think I almost understood why.