Trisha Brown, Sadler's Wells, London

Trisha Brown's 20-year-old Set and Reset returns to London with the reputation of a masterpiece. It's a dazzling collaboration between Brown, her designer and composer, and it's the only thing worth watching on her bill at Sadler's Wells. For a great dance, though, it seems a little fragile.

Brown is one of the New York post-moderns of the Sixties. She staged minimalist pieces in which, held up by ropes, she and her dancers walked on walls. (There's a little wall-walking left over at the start of Set and Reset, one dancer held up by the others.) By 1983, she had moved to more conventional stages, and invited more of the New York avant-garde to collaborate.

The first thing you see is Rauschenberg's set, a box and two pyramids of ice-white gauze. Black-and-white film is projected on to all three, and the images seem to flicker inside those translucent shapes. Like the sound collage that opens Laurie Anderson's score, the images are random and highly organised. Rauschenberg's installation is lifted overhead while the dance begins, but it's hard to take your eyes off it.

Brown's dance can look as shadowy as those film projections. In fact, it's a highly structured piece, steps built into patterns. There are cartwheels, dives into the arms of other dancers, lifts that change direction mid-air. None of this is dramatic: Brown's style is an attack so light it looks casual.

The dancers shrug their way through those steps, with no more emphasis than they put on the little dips, turns of the knee, loosey-goosey arms. It's almost inconsequential; the dancing can look slight to the point of vanishing. But it does hold its own against that Rauschenberg set, against Anderson's insistent bells and synthesisers. Those lightly weighted steps and patterns stay in the memory.

The rest of the evening doesn't. In Geometry of Quiet, dancers draw long white draperies across the stage, pull them taut, dance behind them. Others stand still, or inch their way from one step to the next. It's very, very slow, and there's nothing in Brown's style to help her dancers sustain it.

That loose, casual movement doesn't project very far. When Brown's dancers stop still, there's nothing in their pose or bearing to make them interesting. The slow steps don't flow, because they can't keep movement going for that length of time. It looks as if they're counting how many beats each tiny step should take.

Salvatore Sciarrino's score is a collage of wind sounds, a piping flute and resonant jars. It sounds desolate, and serves to make the dance look even more forlorn. Groove and Countermove is faster but no more gripping. Dave Douglas's summery jazz score would sound right on a movie soundtrack. Brown's onstage action isn't active enough.

In a long opening, two women jog around, setting restless patterns to Douglas's beat. The music gets harder, higher-pitched and frenetic, and Brown brings on more dancers and faster steps. Their clothes, unflatteringly cut, are different bright colours, and Brown lines them up, a rainbow of fruit gums working in canon.

It looks rather good in the production's publicity shots, but it's a letdown on stage. The idea promises a bold image, and we never quite get to it. The dancing, as weightless as ever, just looks messy.