“I'm a harmless clown,” says the plump, philosophical Irishman whom Kitty, the fraught heroine of Lucinda Coxon's new play, encounters at a business conference.

But this stranger is anything but harmless. With his smooth-talking intensity and air of affable self-deprecation, he's Mephistopheles, his offer of no-strings sex and recognition of Kitty's loneliness and dissatisfaction dangerously alerting her to everything that's wrong with her well-heeled life of married motherhood and career high-flying.

Initially Coxon's writing looks like sketch comedy rather than drama. But Thea Sharrock's whip-smart, painfully funny production becomes increasingly cutting, exposing the casual mysogyny commonplace in our supposedly post-feminist world and the con of fairytale romance and marital happily-ever-afters. Once the children are in bed and the obligatory story - Sleeping Beauty - has been read, husband Johnny and Kitty, played with waspish wit by a hollow-eyed Olivia Williams, throw and attend dinner parties where the cutlery is as likely to spear a jugular as an hors d'oeuvre.

Johnny's bitter, alcoholic friend Miles (Dominic Rowan, so brilliantly obnoxious that he makes your teeth involuntarily grind) baits his wife Bea, who is expensively redecorating their home, burying her misery in endless, indistinguishable paint charts. They all envy what they mistakenly see as the freewheeling sexual hedonism of Carl, Kitty's gay friend. And if Kitty's high-ranking, well-paid job at a cancer charity highlights her dual role as nurturer and main breadwinner, Johnny's - as a teacher at a comprehensive school - is just as crucial to his identity as a liberal making a meaningful social contribution.

Meanwhile, Kitty's father is dying in hospital and her self-absorbed mother, whom he walked out on decades before, is still trying to score points. Dad is threatened with amputation; mum - a perfectly coiffed, maddening, callously genteel Anne Reid - develops a cough. His aorta bursts, she gets toothache. It's a nastily precise picture of the way in which intense, long-lived intimacy can become equally enduring enmity. Still more chilling is a scene when, in Kitty's kitchen, Johnny, Miles and Carl all turn on her in an exchange of misogynistic barbs disguised as laddish banter.

Coxon could draw the threads of her argument together more decisively, and her play needs a stronger conclusion; in the end, it does little more than make a series of not unfamiliar observations. Her gaze, though, is unwavering and merciless, and Sharrock's production an excruciating pleasure.

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