IF THERE was ever a ballet weighed down by expectation, this was it: a big new work by a big-name American choreographer for a company badly in need of a hit. Twyla Tharp's Mr Worldly Wise was custom-built for the Royal Ballet, a vehicle both for its handsome corps and some of its brightest stars. The stage was set for a triumph. Curious then, that in the lull following Act I at the world premiere last Saturday, and again after Act II, there was a palpable lack of buzz. Whatever had been expected (something more modern, more bold, more difficult?) it clearly wasn't this.

Tharp's ballet is an allegory in the manner of Pilgrim's Progress, from which she lifts the name of the title role. There is but the slenderest thread of a story. Rather, the work is conceived like a painted church screen, a morality tableau in three parts: Act I, the sinner's despair; Act II, a glimpse of Paradise; Act III, redemption. The work also alludes to the creative artist's dilemma: to surrender to the demons of egocentricity and excess, or pursue discipline and self-restraint. The entire evening is set to a selection of music by Rossini whose life as wildman-reformed provides a tidy parallel. We may also fancy a parallel with the colourful career of Tharp herself.

The curtain rises on Irek Mukhamedov's shock-haired, unbuttoned vagabond Mr Worldly Wise and his sidekick Mister Bring-the-Bag (Tetsuya Kumakawa). Their intemperate life is suggested by an infernal galop of freakish creatures backstage - mad hatters, march hares, a chorus of hip-twitching nuns, even a selection of dancing vegetables - all products of Mr W's rapacious imagination which both enthral and torment him in their whirling dance. He can't keep up the pace. His life is on the skids. The score, having romped through three vivacious Rossini overtures on the trot, breaks into the painful yowling of "The Cats Duet".

The master/servant relationship underlines this disintegration in a slapstick double-act which leaves Mukhamedov with egg on his face. In a series of competitive balletic gags, the diminutive Japanese has the natural edge - leaping higher, twiddling his toes more wickedly, spinning neatly like a top while Mukhamedov topples and stumbles. Here Tharp is playing with fire. Can she be sure everyone knows that he's not tripping up for real? We did not come to see the Royal's greatest dancer as the butt of a circus act. We imagine he'll come into his own in Act II.

Sadly not. For its entirety he sits and watches as Darcey Bussell, Mistress Truth-on-Toe, introduces her ambrosial realm of calm, order and propriety. Under an unchanging white light, ranks of dancers in tennis clothes busy themselves in routines which, though incorporating some jolly moves derived from tap and jazz, suggest some kind of celestial work ethic. No rumbustious orchestral music now. For 30 minutes all we hear is the thin tinkle of Rossini's piano music - the sort of thing they use in ballet classes. An inventive quartet, and some grandly spacious solos for Darcey Bussell, cannot prevent the unholy thought that Mr Worldly Wise would be bored rigid in this company. The devil always has had the best music (and the best dancing parties), and we rather hope our hero won't desert him for this bloodless muse.

Here is one flaw in Tharp's dramatic structure. She also keeps us waiting an unconscionably long time to see Mukhamedov really dance (we have him finally, body and soul). But perhaps this expectation is misplaced when conventional development has been discarded. We are in uncharted territory and must learn to map it for ourselves. Tharp does briefly reconcile the extremes of the first two acts: carnality and conscience come together when Mr W manages to engage Mistress T in an impassioned duet, but afterwards, her mind on higher things, she flits off leaving him once more tormented. The extended solo that follows allows Mukhamedov to mine his huge dramatic gift, but ultimately leaves his predicament unresolved.

The production has much to admire in its elegant complexity, its joyous designs by David Roger and superbly polished dancing. On its second season at Covent Garden it may seem less of a quirky beast.

Twyla Tharp: ROH, WC2 (0171 304 4000), 18 & 20 Dec, 3 & 4 Jan.