In which Marie (Caroline Ducey), cheesed off at live-in boyfriend Paul (Sagamore Stévenin) because he's suddenly more taken with Charles Bukowski than her, embarks on a personal sexual odyssey. That old chestnut again. Except, unlike Tom Cruise's supposed journey to the outer limits in the listless, limp-wristed Eyes Wide Shut, Marie gets fucked. Up there, on screen, 24 frames per second. Which at least makes Catherine Breillat's feature the most explicit to reach a panting British audience since Oshmia's celebrated Ai No Corrida: few punches are pulled.

First there is Paolo (Italian porn stud Roco Siffredi), the handsome widower the twenty-something school teacher picks up in a bar. Then her own headmaster (François Bérland), who introduces the initially sceptical Marie to the joys of bondage. And that before she's anally raped by a piece of street trash and finally impregnated by the no less distracted Paul.

All of which is pretty much French Art Movie by numbers. The film starts with Paul and Marie arguing over the way his initial enthusiasm always turns to self-imposed celibacy. That's how I am, he says. Well, that's not how I am, she retorts. The problem for us is that Breillat's screenplay is weak and banal. Her actors mouth lines - "I don't like guys who screw me. I detest them. I want to be a hole, a void" - that play like pastiche. "It's metaphysical," Marie explains at one point. "I disappear in proportion to the cock taking me. I hollow myself out. It's my purity." Her audience stifle a snigger.

The first half of Romance is frankly awful, the sex - explicit and unsimulated - embarrassing because it lacks context. It has neither the keep-going-at-all-costs close-up athleticism of honest hardcore, nor the demure intellectualism suggested by the presence of subtitles. The scenarios that give birth to the humping are no less ridiculous than standard porn flick 'plots'. It's only in the later stages that Breillat finally locates something more satisfying. The bondage scenes, for example, are played out with a formal grace, as much ritual as sex; they would not look out of place in, say, the cinema of Atom Egoyan or Peter Greenaway.

It's the final third, however, that justifies our interest. If the first hour is excruciating (for all the wrong reasons), then the last thirty minutes belong to a different film. If the remainder reminds us - however vaguely - of Catherine Deneuve's sexually adventurous housewife in Buñuel's classic Belle de Jour, then the closing passages of Romance transmute from glum French bonk-a-thon into a full-blown Buñuelian template. Marie's fantasy of an anonymous brothel (crotches protrude through portholes, at the willing service of male clients), her squirm-inducing internal examination by a parade of blank-eyed medical students and the final funeral scene are all seen through the master's playful surrealist eye. Her revenge on Paul, mind, is pure Almodovar.

Suddenly our interest is peaked and Breillat's film finds its second wind. It's fast and furious and queasy and irresponsible. Yes, irresponsible. That's something the remainder could never be. It's as though she is scared to let go. Too many formalist games, always one eye to the censor. The last half hour is substantially different. The whole is better than the Kubrick because at least it's half as long and infinitely dirtier. But it has none of Oshima's danger because it never turns its audience into voyeurs: everything is exorcised for us, not furtively glimpsed by us. It shows sex but never takes aim on taboo. And that, however explicit Romance gets, is ultimately cowardice.