The Wonderful World of Dissocia, Royal Lyceum Theatre, Edinburgh

Your nerve endings jangle right from the start when Lisa, numbly twanging and tuning her guitar, screws the peg until the top string snaps.

Your nerve endings jangle right from the start when Lisa, numbly twanging and tuning her guitar, screws the peg until the top string snaps. We watch her from a distance, knowing that sooner or later something has got to give, and that when the safety curtain behind her rises on Dissocia - be it a place or a state of mind - she will surely encounter something strange and surreal. After all, this Edinburgh Festival commission from the ground-breaking, Edinburgh-born writer Anthony Neilson is supported by the Scottish Executive's national programme for improving mental health and wellbeing. The clues couldn't be clearer.

After a visit from a gnomic Swiss watchmaker, alarm bells ring when Lisa discovers that she's all out of sorts because she lost an hour somewhere over the Greenwich meridian. Her quest for its recovery takes her shooting off into the so-called wonderful world of dissocia. There she encounters a kaleidoscopic cast of pantomime characters. The Wizard of Oz with some sex and violence, Alice in Wonderland with songs, and a shot or two of spoof sci-fi - the allusions are endless.

The carnival-like atmosphere of the first part of the play is largely fun and games as Lisa's jumbled brain strives to make sense of insecurity guards, a vicious scapegoat and a comforting polar bear. Then there's the lost-property office where the mirthlessly laughing man has lost his sense of humour, the naked man has lost his inhibitions and everyone has lost their marbles. Over this kingdom, where jolly songs cheer Lisa along, hangs the shadow of the Black Dog. Dark threats of war and sexual assault are projected alongside the comedy.

The second part switches to the bleak whiteness of a hospital ward, the action set behind a glass screen. From behind it every sound is spookily amplified. Lisa, sprawled on a bed, encounters the other side of dissocia, as a constant stream of detached nurses and doctors apply their medical knowledge to another "case".

Of course, the harrowing symptoms of any mental illness - dissociative identity disorder in this case - cause anguish not only to sufferers but also to those close to them. In Lisa's case these include surprisingly few: just her sister, frustrated into a cold lack of sympathy, and her boyfriend, driven into the near-termination of their relationship. Lisa is brilliantly portrayed by Christine Entwisle, the constant tangled thread in this focus on the prejudices that colour our attitude to mental health.

That Neilson's daring approach works so well and on such different levels is as much down to the whole colourful and communicative cast as to his own boundless imagination.

Continues at the Drum Theatre, Plymouth (01752 267222) 9-25 September