The idea of asking a slew of comedians to retell the same joke over and over again for 89 minutes is an intriguing one. You'd think it must be a good joke. Actually, there's not much of a joke to tell. It's really about how much each comedian can outfilth the other.

The publicity blurb tells us that the joke has been with comics since Vaudeville, but is so offensive that it has never been told in public. "It's a secret handshake among comics." For a while I was wondering whether the vehicle for this documentary was a clever invention and all the comedians were in on the secret, but as the roll call continued it seems that would be too elaborate.

Being filthy dirty is not funny (as my mother always used to tell me when she caught me using too many four-lettered words), at least not by itself. Delivery and patter makes all the difference. If it was that easy to get a laugh from a few "mauvaises mots," I'd spend a lot more time eavesdropping on building sites.

This is about how comedians take the joke and give it their own twist. Director Paul Provenza, who made the film with magician Penn Jillette (of Penn & Teller), compares the art to free jazz. You take a melody and you embellish.

For someone not too familiar with the American stand-up scene, it was interesting to see some lesser-knowns riff on the theme, alongside Whoopi Goldberg, Robin Williams, Billy Connolly and a host of other big names. The style ranges from the ballsy to the weird, from the utterly camp to the sexily svelte.

I found myself laughing till I cried at times, wincing (often at the same time) and checking with my neighbour to see if that is what they really said. That people walk out at screenings is no surprise. When there's any danger of the pace cooling, another shovel of excrement, or after-birth humour is layered on.

The doc is elegantly structured, if too l o n g. At times the production values are as shockingly crude as the language and there are quite a few out-of-focus, low-lit interviews, during which the camera appears to be inside the comedian's nostrils. Judging by the number of microphone wires showing, the filmmakers just grabbed people opportunistically. For instance, Eric Idle is memorably poorly filmed.

There is also a focus group element, where the writers of satirical mag The Onion deconstruct the joke, which seems superfluous, since many of the comedians themselves talk very eloquently about how they approach and then burst through the boundaries of good taste. Pretty much everything seems ripe for dredging through the sewer, although racial humour is still a dense minefield. Bernard Manning would not be welcome here.