Before I Sleep, Old Co-op Building, Brighton Festival

There's a sign at the end of Dreamthinkspeak's new work, a promenade piece in a defunct department store, which implores the departing audience not to tell others about the contents of the show lest they give too much away.

Clearly this poses something of a problem for the reviewer, most notably how to convey the magic of this work without spoiling it. All I can say is that Before I Sleep is the most mesmerising, atmospheric, visually exquisite piece you are likely to see at this festival, or indeed anywhere.

Dreamthinkspeak have been quietly reinventing site-specific theatre for some years now, creating walk-through shows all over the world, from a Moscow paper factory to a hotel in Kuala Lumpur. Their latest piece takes as its inspiration Chekhov's The Cherry Orchard, a story about a debt-ridden aristocratic family who sell their estate to a serf-turned-entrepreneur. Much of the narrative here falls on Firs, the old footman who remains on the estate and who, in the terrible final act, finds that the family have left without him and boarded him up inside the house.

Thus it is Firs whom you meet at the start, a stooped, scraggy man who stares from a doorway and jabbers at you in Russian before beckoning you into the dark. From there you are directed through a series of rooms containing images of immense, arresting power.

Taking in film, music, performance and installation art, Before I Sleep is far from the traditional theatre experience. It is, in effect, a series of set pieces that rely on sound and smell as well as visual and narrative flourishes to propel the viewer forward.

Dreamthinkspeak's request to keep schtum about what takes place is a difficult one since when you exit the building, it's all you can do not to stop strangers on the street and harangue them with the details. Best go soon, then, before the secret gets out.

To 23 May (Brightonfestival.org)