Documentarist Terry Zwigoff's second film carries at its front the credit "David Lynch presents". At first it's easy to see why - there is enough weirdness bobbing in the Crumb family sea to buy them a condo in Eraserhead-land. Seething darkness in the bilious belly of the American Dream, kind of thing. The difference here is that everything is for real.

Robert Crumb is possibly the world's most famous comics artist, even if the things he's famous for - Keep On Trucking, Fritz the Cat - he all but disowns. (Early in the film he shows them to a college audience as both introduction and as a way to distance himself from them.) Intricate, obsessive, offensive, as soon as his Rotring hits paper Crumb provokes a response. Is the work misogynist? Quite possibly - he admits as much on camera. Racist? He thinks not, although some might disagree over Ooga Booga. And what he intends with his infamous strip pushing incest as a cure-all for family ills is anyone's guess. The suspicion is that Crumb himself has absolutely no idea.

The film is ostensibly about the unrepentantly geeky Robert, his work, wives, masturbation (five times daily to his own pictures; he has, says a girlfriend, one of the biggest penises in the world), a childhood fixation on Bugs Bunny and a hefty collection of jazz and blues 78s. But to understand the man Zwigoff has elected to go to family Crumb, opening a whole can of grotesque worms at which Robert himself merely hints.

Brother Charles is a reclusive manic depressive who lives with their mother Beatrice and spends his days rereading the books he read as a child. A proficient cartoonist and handsome teen, an obsession (sexual, of course) with of all things Treasure Island eventually matured into graphomania, filling book upon book with meaningless word-like scrawl. And yet Charles is at least aware of his own desperation and speaks of wanting to escape even if both he and the audience know it is not about to happen. (Lynch was reputedly so moved he wanted to make own film.)

Younger brother Maxon lives in a squalid San Francisco hotel, painting and practising yoga on a bed of nails. He has a history of molesting women. Like Charles, Max is ready to critique his situation, and yet unlike Robert seems unable to make the break. His paintings are accomplished and outlandish. (The two Crumb sisters refused involvement in the film.)

Around this pair Robert is at his most normal, laughing at their stories, reminiscing, sympathising. Around others he is always trying - smirking and timid - so much so that an ex-girlfriend, discoursing on his innate emotional dishonesty, feels moved to hit the grinning fool. It's an unconscious mask that only really falls when he's asked to take part in a shoot for porn mag Legs Show, run by another ex-girlfriend; surrounded by semi-clad buxom high schoolers he's like a kid in a candy shop. (The scenes with his own young daughter are entirely innocent, but extraordinarily charged.)

The film unwinds with a rich humour and sense of impending tragedy that fiction simply cannot provide, as though waiting on something awful to happen. And then, in the final seconds, it does. "I'm nauseous" is how Crumb describes the whole process at the end as he and wife Aline prepare to live in France. Mortified by the film, communications were apparently frosty, according to Zwigoff, and Crumb now sports a beard disguise. Whatever the rights and wrongs, this is one of the funniest, most honest and plain haunting films of the year.