Not since Blue/Orange, and its penetrating debate about the diagnosis and treatment of a mentally ill man, has a play both excited and disturbed me as much Boy Gets Girl.

This American play - originally seen in Chicago and New York last year and now receiving its British premiere - is the second by Rebecca Gilman to be presented at the Royal Court this year alone. In January, the Theatre Upstairs offered Gilman's Spinning into Butter, an evocative and provocative exploration of racism on an American university campus.

The theatre's advocacy of someone who is clearly turning out to be a major new playwriting voice proves not to be misplaced. Now graduating to the mainhouse Theatre Downstairs, Gilman's Boy Gets Girl belies the suggestion of its title that it will be a light romantic comedy to offer a far more aggressive, darkly disturbing portrait of a Manhattan career woman, Theresa Bedell, who becomes the victim of a stalker.

As charted with an almost documentary-like realism, we're shown Theresa's innocuous first blind-date drink with Tony, a friend of a friend, and the subsequent dinner date they arrange - after which, Theresa politely rebuffs Tony's further attentions. But rather than being deterred, he begins a relentless pursuit: flowers start arriving; he phones; he turns up unannounced at her office; he keeps calling, both at work and at home. Then it turns nasty.

What starts out creepy becomes gradually terrifying. Gilman not only notches up the tension acutely in what is in a sense a psychological thriller, but also provides detail and debate to give it real dramatic life.

Theresa's safety, home, career, and even identity are at stake, and the play is remorseless in showing just what that means. As these tensely compelling events unfold, we are grim observers of her collapsing world - eager, as her sensitive male work colleagues are, to help, but powerless to do so.

In Ian Rickson's riveting, fluid production, the cautionary tale is acted out with fierce and wounding commitment throughout. As Theresa, Katrin Cartlidge gives one of the performances of the year, and there's also a superbly funny cameo turn from Lucy Punch (last seen as the winsome daughter in The Graduate), as her well-meaning but clumsy secretary, Harriet.

This is one not to be missed.

- Mark Shenton