PARK CITY - Reviewing a new Lars von Trier joint is never exactly a breeze, though it's usually a little easier when you've seen the whole thing. Presented last night as the not-so-secret Secret Screening at the Sundance Film Festival, "Nymphomaniac (Part One)" offers at least a full film's worth of theories, provocations and retina-branding images in its first half -- as well it might, given that its first half is nearly two hours long. But they're cut off cruelly in limbo, with nothing so much as a tidy temporary knot or mini-catharsis to tide us over until Part Two, and I can't feign any insight as to the film's narrative or thematic endgame.

This is not, of course, how von Trier intended his film to be seen. Public audiences in Europe (plus selected critics wortldwide) have been wallowing in the four-hour pairing of Parts One and Two since Christmas. And that marathon still represents a curtailment  of von Trier's full, 330-minute vision, the first part of which will be unveiled at Berlin next month. When, if ever, Lars-lovers will be able to absorb the sprawl in one sitting remains to be seen, but for the purposes of US cinemagoers, "Nymphomaniac" will, for better or worse, be viewed as two movies.

Whatever fraction of "Nymphomaniac" we were treated to last night in Park City, it doesn't feel perceptibly shortened, compromised or darned -- that abrupt, inevitable break-off point notwithstanding, of course. Its shape is elegant, its rhythms confidently off-kilter, its discussion articulate -- loopy, of course, but articulate. (We're talking about a film that relates sexual discovery to fly-fishing and Fibonacci numbers, and that's just in the first 10 minutes.) Whatever else has been lost in the cutting room, it's not the formal grandeur or rhetorical fury that distinguish any von Trier film from, well, anything else. It's not his knack for drawing fiercely crazed, open-wound performances from his stars -- his women, in particular. And it's certainly not his splendidly grim sense of humor: bursting with salty dialogue and exquisitely timed comic music cues, it may just be the funniest film the director has ever made.

It's also, at least at the halfway mark, another wincingly impressive showcase for Charlotte Gainsbourg. Now in her third successive film for von Trier, she has convincingly bucked the director's infamous one-and-done record with his leading ladies, and appears to be settling as just the right counterbalancing muse -- delicate, deliberate, dryly intelligent -- for von Trier's brash, both-feet-forward sensibility.

In a characterization that hews closer to the terse, self-punishing mother of "Antichrist" than the fretting conscience of "Melancholia," Gainsbourg plays Joe (yes, with an "e," for whatever symbolic significance that's worth), a young woman whom we first encounter bruised and bloodied -- yet serene enough to request only a cup of tea when help is offered -- in a back alley. (I think we're in London, though VonTrierVille is as geographically imprecise as ever.) A kindly but eccentric stranger (fellow von Trier regular Stellan Skarsgard) invites her back to recuperate at his apartment, where she relates her life story in the manner of a particularly pervy fireside tale. (Yes, I know I only recently warned against the perils of films with chapter headings, but von Trier gets away with any number of affectations he shouldn't.)

Joe is, as it turns out, the chronic sex addict of the title, and her affliction has shaped her life from infanthood: "I discovered my c--- at two years old" is a line few but von Trier would write, and few but Gainsbourg could deliver with such soft matter-of-factness. The young Joe (bright new find Stacy Martin, an eerie physical and vocal match for Gainsbourg) discovers considerably more than that as she reaches adolescence, and her virginity is roughly taken by Jerome (an amused-looking Shia LaBeouf, his accent pitched halfway between Beatle and Beantown), an older boy who looks to be a recurring weakness in her life.

Having acquired a taste for intercourse, then, Joe sets about finding as much of it as humanly possible, setting up a busy daily rota of sex buddies and employing their physical services without regret or emotional commitment. Not all her partners manage quite the same detachment: cue the film's single most startling (yet fully clothed) sequence, in which the discarded wife (Uma Thurman) of a delusional john visits Joe's apartment with her three young sons in tow, curtly requesting that the boys be allowed to view "the whoring bed." It's a small performance of riotous venom from Thurman -- possibly the 10 finest minutes of her career -- that calls for a dedicated lead role from von Trier, should Gainsbourg ever relinquish the privilege.

These grim confessions -- including a harrowing monochrome recollection of the mental and physical breakdown endured by her father (Christian Slater), in which not even gallows humor is deployed -- are calmly absorbed by Skarsgard's listener, whose bizarre non-sequitur responses ("The biggest fish choose the best positions," he intones, doggedly sticking by his angling metaphor) may or may not reflect our general awkwardness in matters of carnal discussion.

But truth emerges from this mannered conversation too: von Trier is a blunt writer, but never a glib one, and the film's best exchanges crackle with sexual, philosophical and even theological insight. Not as explicit upfront as we'd been led to expect -- I'm told things get kinkier in Part Two -- "Nymphomaniac (Part One)" is, somewhat wittily, about the value of talk as much as, er, action. Shot with crisp autumnal elegance by Manuel Alberto Claro, it's a graceful, thoughtful human comedy that just happens occasionally to lapse into Rammstein-scored chaos and extended montages of flaccid male genitalia -- just as any warm-blooded being occasionally gives in to their filthy side. Two hours, at this point, is far from enough.

Guy Lodge is a South African-born critic and sometime screenwriter. In addition to his work at In Contention, he is a freelance contributor to Variety, Time Out, Empire and The Guardian. He lives well beyond his means in London.