THE JOKE THAT is the ostensible subject of the obscene and at times hilarious documentary "The Aristocrats" -- and whose punch line lends the film its deceptively innocuous title -- is not an especially good one. It does, however, provide a marvelous opportunity to take a look inside the psyche of the working comedian, more than 100 examples of whom were persuaded to come before the camera of director/stand-up comic Paul Provenza and his partner in crime, executive producer Penn Jillette, to tell the same dirty gag (and I mean gag) over and over and over.

After a few such peeks into their cobwebbed, crust-encumbered craniums, you may want to slam the lid shut and run for the nearest hot, soapy shower.

The story -- circulated backstage and after-hours by comics since the days of vaudeville, yet virtually unheard by paying audiences -- goes like this: An entertainer approaches a talent agent with a pitch for a new act, featuring his family members. "What kind of performance does your family do?" asks the agent, who is then told (or just as likely shown) a graphic series of increasingly horrifying behavior, sometimes set to music and typically involving a prolific combination of incest, scatology, violence, rape, vomiting, bestiality, necrophilia, torture, physical contortion, mutilation, child abuse, masturbation, incontinence, Liza Minnelli and the pope (the last two being entirely optional). "And what do you call yourselves?" asks the shocked agent. "The Aristocrats," comes the jaunty reply. If it helps, try to imagine Drew Carey spicing up the delivery of the kicker with a flamenco-style finger snap.

See what I mean? Not very funny.

Yet if the joke beggars description, or even analysis, especially in a family newspaper, it truly comes alive in the telling, the aim of which is essentially to blow a hole in propriety through which you could drive a garbage truck. It gets to the funny bone not with a feather but with a flame-thrower. It even, in its unsubtle way, requires a certain artistry.

Several of the versions of the joke heard in the film, which features such comics as Whoopi Goldberg, Robin Williams, Merrill Markoe and Emo Philips, are particularly entertaining. Among the more memorable renditions are those by the formerly family-friendly Bob Saget, who breaks up in laughter every few sentences while questioning the very wisdom of his participation, yet gamely forges onward, and by Sarah Silverman, who delivers the tale not in the standard guy-walks-into-a-bar format, but as if dredging up the memory of a long-suppressed episode from her own childhood. It's a brilliant, disturbing and, yes, riotously funny piece of spontaneous theater.

It's a good thing that the movie is on the short side, because after a while, the unrelenting verbal barrage of vulgarity, blasphemy, immorality and illegality wears thin, especially when you consider that a core part of the joke's raison d'etre is to out-gross-out the guy (or the girl) who told it before you. What keeps "The Aristocrats" from becoming a deadening exercise in one-upmanship, however, is the explicit and implicit commentary offered by the film and its preternaturally articulate participants on the psychological roots of comedy in pain.

Under normal circumstances, nothing kills a joke faster than trying to explain it. Yet here, such examination is the film's strong suit and provides much-needed respite, quite frankly, from the exhaustion of constant laughter. Humor, it is often said, is thinly veiled aggression, yet what becomes clear is that there is something therapeutic, too, both for the artist and the audience. What the subtext of "The Aristocrats" shows, and what Silverman's tour de force is an exemplar of, is that not only can you laugh so hard it hurts but that, even under the most blistering assault on decency, you can laugh so hard it heals.

THE ARISTOCRATS (Unrated, 87 minutes) -- Contains numerous foul-mouthed interpretations of a single filthy joke. At Landmark's E Street Cinema, Loews Georgetown and Muvico Egyptian Theatres.

Whoopi Goldberg, one of more than 100 comedians who appear in "The Aristocrats."Andy Dick and others try to outdo one another, telling their versions of a classic dirty joke.The one-joke movie, with Gilbert Gottfried, is -- funnily enough -- chock-full of laughs.Richard Lewis and his colleagues break down the joke without losing its sense of humor.The documentary, directed by comic Paul Provenza, features the legendary Phyllis Diller.Bob Saget's telling of the joke will derail any thought of him being "family friendly."