Sex sells. Sex on stage sells out. And as the public relations adviser for the play Specialconfidently informed me, “put S&M in the title and you are guaranteed a full house”. It’s probably true, but at the Edinburgh Festival the number of productions that opportunistically employ shock sex tactics to get bums on seats means that pretty much every sacred cow has already been slaughtered. From bestiality to topless women in wheel-chairs, it’s all been done before with varying degrees of success, and, somewhat predictably, when you type S&M into this year’s Edinburgh Festival search engine, Special is not the only show to come up. However, I suspect it is the one worth seeing.

Tackling what is still one of the most demonised forms of sexuality is a masochistic challenge for any writer or director, but John Keates has managed to steer his story clear of predictable clichés by containing the sexual exploration within the confines of a very “normal” domestic relationship. By avoiding the stereotypes of rubber clubs and torture gardens, the audience can identify with what is happening on stage in a way that would not otherwise be possible.

Through the course of the play, two characters, Emily and Steve, hang out in their living room, drink G&T, act out their S&M fantasies, get married and get pregnant. Emily is the “top”, the dominant one. Candle wax, thumb tacks, clothes pegs and deep heat are the tools of her trade and the contrast between her sadistic sexual tastes and her maternal instinct make uncomfortable viewing. In Special, the fantasy roles of bondage, pain, punishment and submission seem to have no place in a relationship that confounds us with its ordinariness. Though we know S&M happens, we like to keep it at arm’s length. The dominatrix, for example, is usually reduced to a kind of caricature Miss Whiplash so that she can be easily separated from “normal” women. By forcing us to face the sight of a functional, articulate, happily married woman taking pleasure from walking on her husband’s testicles wearing stilettos, Keates leaves us feeling conflicted.

S&M may not be a mainstream sexual practice, but there are millions of web pages devoted to the subject and images of female fetish and bondage are employed in advertising all the time. Verifiable statistics are hard to find, but it is estimated that 14 per cent of men and 11 per cent of women have engaged in BDSM (bondage, domination, sadism, masochism). It is hard for someone who has never experienced pain as an erotic pleasure to understand the attraction of S&M, but Derek Cohen, the chairman of the Spanner Trust, puts it well: “If I were to run my fingers down your back it would be a pleasant experience. If I were to use my nails you might enjoy that, too. If I were to press harder still you might find it too painful, but another person might enjoy the intensity of the sensation.

“How you perceive pain is largely contextual. If a person is being punished against their will then it is pure pain, but if a person is being punished in a consensual power exchange the pain is eroticised.”

People find S&M relationships confusing because they defy traditional assumptions about power within a partnership. As part of his research Keates visited a dominatrix who explained that, contrary to popular opinion, S&M is not just about the dominant person getting what he or she wants; it is about power sharing.

In a consensual S&M relationship there is no abuse, no violence and no lasting injury. Couples agree code words or gestures in advance to signal when the pain becomes too intense, and this means that the needs and desires of the submissive are actually the priority. The power exchange is intellectual, too. When Steve says “She lets me embrace my failings and after that my daily life is easy” he explains how, for many submissives, S&M play is true freedom, a holiday from control and responsibility.

Fortunately, Keates doesn’t attempt to explain any of these technicalities.

Had he fallen victim to the understandable urge to educate his audience, or tried to create a representation of what he thought an S&M couple might look and sound like, Special would be anything but. Instead Keates arrived at the existing script through an intense period of improvisation, a process that allowed him and his acting partner to work out the physical, emotional and intellectual responses that a couple engaged in S&M might experience. They spent several weeks locked in a hotel room working out what actions and exchanges heightened the charge between them. Keates believes that it is this process of personal exploration that will give Emily and Steve’s relationship credible intensity and, hopefully, he says with a grin, “leave the audience feeling intellectually, and, hell, why not, physically stimulated”.

Special, Assembly Universal Arts Theatre, Venue 7, George Street, Edinburgh, (www.assemblyfestival. com 0131-623 3030), Aug 2-27