Robert Wilson's HG frees us from the tyranny of conventional theatre, says Clare Bayley

Robert Wilson, one of the great theatre-makers of this century, frequently proclaims his frustration with theatre as a medium. His work has constantly taken theatre beyond the bounds we recognise (his famous KA MOUNTAIN and GARDenia TERRACE was performed simultaneously on seven hills in Shiraz non-stop for seven days) and with HG he goes further down the road of mixed-media installation than anyone else to date. It's a road that is beckoning more and more of our most exciting theatre practitioners - Deborah Warner and Hildegard Bechtler's St Pancras Project earlier this year was a significantly mainstream bridge between installation and theatre, and Tilda Swinton has just linked up with Cornelia Parker for The Maybe at the Serpentine - and the more this path is explored, the more audiences can learn about their response to both media, as well as the hybrid offspring.

HG plunders theatre's box of tricks without technically being theatre. There is no storyline, no script and no actual characters. The HG of the title, whose engraved cigarette case and other paraphernalia are scattered around the installation, never appears alive. Yet the experience of HG is fundamentally theatrical; you volunteer to go into a dark space carrying with you certain half-formed expectations. You are presented with visual, auditory and sensual images and clues: Hans Peter Kuhn's sumptuous lighting and sound, a human mummy lying in a shaft of white light, an unseen whistler who is disconcertingly close to your ear, a glow of purplish light through which a shadowy figure seems to materialise and dematerialise before your eyes.

You willingly enter into the fictional world that is created for you and employ your imaginative powers to make sense of it. Sometimes you experience fear, sometimes sorrow, occasionally you laugh. Your fellow audience members have similar experiences, though not necessarily at the same time. You may start to put the clues together and invent a story connecting the images, though you don't have to and there isn't a "right" or "wrong" solution.

Some commentators have likened HG to a series of film sets deserted by the actors, which may be a useful guide to how to approach it but entirely misses its potency. You are told by the woman on the door that, "The year is 1895 and something has just happened." And when you enter the lavish dining-room where candles glitter and the gamey smelling remains of a dinner party coagulate on plates, it isn't a cinematic experience, it's real and sensual.

The whole power of theatre, which many still forget (including the Channel 4 Blow Your Mind series on telly), is its real presence: presence of actors and of audience and of the objects on stage. The great double bluff of both Robert Wilson and Deborah Warner has been to create a sense of presence through absence. In the huge, lonely St Pancras Hotel through which each audience member had to walk alone and trembling, Warner employed actors to give an occasional startling glimpse of a real person (a maid or a bellboy) while leaving the rest to the empty building. Wilson creates an impression of many different absent people and employs one real person to embody, with stunning plausibility, a ghost.

For The Maybe Cornelia Parker also employed one actor, Tilda Swinton, and obsessively explored absences and presences. The display in glass cabinets of the cast-off possessions of famous people (Winston Churchill's half-smoked cigar) were less evocative than the clothes (Mrs Simpson's ice-skates), but even these were put into the shade by the waxwork-like figure of Swinton herself. The Maybe, though, was a strictly visual arts affair and so a comparatively thin experience compared to either HG or the St Pancras Project.

Ironically HG is far more emotionally engaging than much of Wilson's stage-work. Borrowing from the visual arts tradition, it allows the audience to wander and freely associate while presenting them with something multi- dimensional, existing in time and space. When a theatre audience is freed from the tyranny of thinking that a piece has a single message to convey, their own imaginative powers kick in. If audiences and practitioners can take that lesson back to conventional theatres, then our theatre-going experiences can only blossom. And we won't need TV to tell us to go see a show.

n 'HG' is at the Clink Street Vaults, London (0171-336 6803) to 15 October