Mark Morris's Romeo &amp; Juliet, Barbican Theatre, London <br>Impressing the Czar, Sadler's Wells, London</br>

Adding a happy ending to Shakespeare's great love story is rather sweet. The tragedy is that the result is dull

And Romeo and Juliet lived happily ever after. While you digest that information, consider why a dance iconoclast who until now has avoided straight romance in his work would take on the biggest boy-meets-girl story of them all – a story, what's more, that has spawned many satisfying dance treatments already. What is truly baffling about Mark Morris's Romeo & Juliet, on Motifs of Shakespeare (the title chosen by Sergey Prokofiev for his 1935 ballet score) is not that he thought it might be a good idea to make known a hitherto unknown early draft of that magnificent music. It's that he turns out to be woefully not up to the task.

The upbeat ending, as conceived by Prokofiev and vetoed by Joseph Stalin, is by no means the most perverse element in this three-hour production. The lovers' last-minute reprieve, thanks to the arrival of Friar Laurence, struck me as wacky but rather sweet. Far more surprising is the weakness of storytelling, lack of dramatic tension and mishmash of gestural movement that passes for choreography. In a word – one I never imagined using in relation to Morris – this show is dull.

Much of the problem lies with the dance language. Morris seems to have aimed for a kind of pre-ballet, a vaguely period style suggestive of Shakespeare's day. But typically he can't resist chucking anachronisms into the pot. So we get Romeo and his chums sweeping low, as if flourishing non-existent cloaks, and Verona street folk kicking up their heels in circle dances, alongside Bob Fosse shimmying hands and classical arabesques. The acting is a queasy mix of naturalism and mime, some of the latter so odd as to be indecipherable (what is that round thing Juliet's parents and nurse keep describing?). Worse, though, the movement contains no element that can hope to match the soaring emotional tenor of Prokofiev's music. Inevitably, the balcony and bedroom duets (the bits most people have come for) fall horribly flat. Though danced with a lovely tenderness, they never get off the ground – this in spite of a plethora of lifts, Juliet lifting Romeo more often than he lifts her, Morris ever politically correct.

Nor is his Verona a place we can believe in. Allen Moyer's piazza is an array of architect's wooden models. Interior scenes are blond-wood panelled like a Lehman Brothers boardroom. Friar Laurence's cell looks kitted out by Heal's. The one visually arresting moment is when the lovers, reunited, make their escape to a blue-and-gold painted chamber where the walls dissolve into a star-studded night sky. But any potential poetry is scuppered by the banality that follows, the pair whirling, joined at the wrists, as if in some celestial school playground.

There are, it must be said, some moments of relief. Lauren Grant's diminutive nurse – dwarfed by her strapping charge – is sharply characterised. Amber Darragh, in what might have been inspired cross-gender casting if it had served any purpose, is a hyperactive and hugely likeable Mercutio. You're genuinely sorry when (s)he dies. And Prokofiev's score, played with touching sincerity by a scaled-down London Symphony Orchestra, sets your ears on stalks, listening for digressions from the familiar version. Nothing to get too excited about – it mostly comes down to orchestration – but there is one "new" passage accompanying the death of Tybalt where stabbing strings and bass drum sound like a precursor to Hitchcock's Psycho.

If Morris's show was a scaling down, an antidote came in William Forsythe's maxed-out Impressing the Czar. Rarely has Sadler's Wells' stage so resembled a Notting Hill Carnival riot as in the opening act of this 1988 work, only now getting its UK premiere courtesy of the Royal Ballet of Flanders. Read the programme and you risk being cowed by Derrida and post-structuralism. Better just to sit back and enjoy this jubilantly chaotic take on the entire history of Western art and thought.

To the fragmented strains of a Beethoven string quartet, later mimicked by what sounds like kazoos, a Breugelesque frenzy of disparate activity unfolds. Old Master canvases unfurl as characters in 18th-century dress cavort, a schoolgirl watches TV from a gilded throne, a St Sebastian figure stabs himself with golden arrows, a group in leotards take ballet class and a woman tries to sever her own leg with a pair of giant scissors.

Lots more is happening, but the eye can't take it all in. That's easier in the final act, where the entire cast of 40 erupts into a furious tribal line dance dressed as English schoolgirls. It's mad. Yet it's so savagely energetic that you happily surrender to the madness.

The 'Focus on Forsythe' season resumes 20 April-10 May 2009, booking now at sadlerswells.com