C (venue details) Daily to Aug 25; 9:05pm (1hr10)

THE FOURTH WALL.  That's what thespians call the barrier which separates their characters from the audience; the surface we can see through, though they can't see out.  And from the Ancient Greeks to Shakespeare's "play within a play" and the modern-day Matrix, as long as there's been a fourth wall, playwrights have experimented with breaking it down.

It takes a special kind of chutzpah, though, to work fourth-wall chicanery into what is essentially a farce.

Yet that's exactly what Tom Crawshaw has done in his magnificent Auditorium, an original work for Fringe veterans Three's Company.  As the play begins Yogi, wannabe actor and employee at a second-rate Edinburgh bookshop, is trying to cover up having spent the night with his hapless boss's ex.  But things don't stay that simple... and from the moment Yogi steps through a beaded curtain to find an audience - that's us! - on the other side, we know we're in for an evening of mind-bending, rule-breaking fun.

At first it's quite an intellectual undertaking: there's word-play which could have been taken straight out of Stoppard, existentialist debates, and an hilarious deconstruction of the audience itself.  But as the story wears on, it gets raunchier, funnier and faster, with Crawshaw's script pressing into service all the elements of classic farce.  There are ridiculous disguises, dishevelled clothing, characters hiding in the cupboard... and lots and lots of running in and out of doors.  In an inspired twist, each time a cast member passes through the curtain, they spend a while snooping on their friends behind the "wall" - and if, by the end of the play, I'd lost track of just whose girlfriend was whose, in this case that's surely a positive sign.

But there's much more to this than slap and tickle.  A whole new story emerges with the arrival of suave Inspector Cornell, whose mission it is to close the portal and trap the hapless Yogi on the far side.  Comically sinister, Cornell involves the characters, the audience, and even a stage-hand in his dastardly plan.  Does he succeed?  I couldn't possibly let on - not least because one of Cornell's asides to the audience raises the intriguing possibility that they've rehearsed more than one ending.

The script's the true star of this show, but that takes nothing away from the actors, who are uniformly superb in their vastly varied roles.  And - though the audience was well-seeded with stooges - there's at least a bit of unscripted, quick-witted improvisation.  At the end, as the complex plot finally untangles, the energy dips a little, but this is the only weakness in an otherwise perfectly-balanced gem of a show.

Farce, existentialism, and a dose of the X-Files: I'd never have thought it a winning combination.  But this continually entertaining and warm-hearted play proves that anything's possible - and that Crawshaw, like his characters, is truly in another world.