Aunt Dan and Lemon, Royal Court, London

Bitter words hit the sweet spot

There's a stunning moment in this extraordinary 1985 play by Wallace Shawn – revived on the stage where it was first seen – when Jane Horrocks, addressing us like the lady on Listen with Mother ("Are you sitting comfortably? Then I'll begin") declares that she's given up reading detective novels to read about Nazis killing the Jews instead.

Shawn is a master of putting the unspeakable into the mouths of plausible but slightly warped characters, and sad little Lemon starts by commending the Nazis for achieving much of what they wanted to do and ends by excoriating a deep-rooted hypocrisy in the cult of compassion.

The villainous hero of the piece is Henry Kissinger, no less, whom Lemon's mentor, the curious Aunt Dan, an Oxford academic friend of her Anglophile American father and soft liberal mother, sanctifies as someone who does our dirty work for us by killing peasants in Vietnam. As a beneficiary of a nasty foreign policy, you have no right, says Dan, to complain about the brokers of your own wellbeing and security.

I was agreeably surprised at how the play had retained its bite in this respect. Shawn insists on making links between our creature comforts, the exploitation of others and the end of the civilised world, and the ambition is not always easy to grasp in the theatre; but this play, even more so than his naggingly provocative monologue, The Fever, acts as a catalyst between private instinct and public evidence of spiritual malfunction.

For Lemon, recalling her first hypnotic encounters with Aunt Dan at the age of 11, is drawn not only into the brutal logic of her theories of consuming survival instincts, but also into her dubious and hedonistic life style, witnessing trite social exchanges and then an extended sex scene – presented here as a sort of ominous snuff movie – in which Dan's lesbian friend Mindy (beautifully embodied by Scarlett Johnson) strangles a police informer after a red-hot casual encounter.

The play is a brilliant convocation of monologue, party sketches, nostalgic remembrance and a poignant friendship. Aunt Dan must surely be based on one of Shawn's New Yorker editor dad's eccentric contributors.

Dominic Cooke's production, designed by Lizzie Clachan and lit by Jon Clark, only flops in the bed business; the bed gets in the way of the party scenes and the trap door through the mattress trick simply doesn't work.

Otherwise, this is a compelling, sensitively poetic production, the 12-strong cast admirably led by Horrocks on scintillating, wickedly confidential form, and Lorraine Ashbourne slinky and disarming, if not quite intellectually convincing, as Aunt Dan. Lemon, as Fintan O'Toole correctly observed, is Shawn's Iago, a creature of strangely dislikeable opinions whom we love to get to know in the theatre. A great evening.

To 27 June (020-7565 5000; www.royalcourttheatre.com)