May 23, 1957, and Alan Simpson is imprisoned for producing a play by Tennessee Williams in which the appearance of a condom on stage breeches an outdated 19th-Century law.

October of the same year and the politician Mr Peter Evans hangs himself after compromising pictures are printed in a newspaper.

May 2004 and a flat cap-wearing spokesman for Ireland's Semper Fi theatre company informs a shivering group outside the Aquarium Colonnade toilets that we are to be taken beyond the black and white of the headlines and into a world of grey.

Last staged at the Edinburgh Festival, this sight-specific curiosity is preceded by its reputation.

We are vaguely aware we will be witnessing two sides of a mystery, the audience dividing between the Ladies and Gents toilets, then switching over at half time.

Yet nothing prepares you for the visceral thrill of being cleft from your evening's companion, cramped into a pitch-black toilet where a gruff voice warns you not to move an inch and finally forced, as oily lights begin to flicker, to play the reluctant voyeur in a secret rendezvous.

Despite having had only a day to acquaint themselves with this space, Semper Fe gives monstrous birth to a twilight world in which the flaring of a match is a wonderfully dramatic moment and the soft fall of footsteps enough to momentarily weaken the bladder.

While Paul Walker's ingenious script ensures neither side of the audience is either confused or complacent, the brilliantly cast actors exude so palpable a threat that we shrink back against the grotty urinals for reassurance.

You can feel their breath, and certainly can't escape the venue's homegrown scent, yet the play's atmosphere and detail is thoroughly cinematic.

In the Ladies, a blue evening dress hangs from the grime-streaked wall, enhancing the creeping sense that you're trapped behind the shutters in David Lynch's voyeuristic drama Blue Velvet. In the Gents, I stare straight into the pale-blue eyes of Mr X, which bulge horribly like his Gladstone bag.

I'm reluctant to give away the chilling plot twist so let's just say that in the final moments something resembling a bodily fluid splatters across my face, and that, though I would gladly buy another ticket, I won't be spending a penny here in future.

Ladies and Gents, then, is no gimmick. Semper Fe has proved itself master of a thrilling new genre - scratch 'n' sniff theatre.

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