Grasses of a Thousand Colours, Royal Court Jerwood Theatre Upstairs, London

Are you a cat person? Wallace Shawn certainly is, appearing in his own new play as a cat that got the cream in the shape of Miranda Richardson, Jennifer Tilly and Emily McDonnell – fine and feline actresses – as well as a white whiskery mouser called Blanche. It's Blanche's lot to get drenched in sperm and abused with the music of Mantovani.

The title is invoked in an epigraph (spuriously attributed to the non-existent "Count D'Aurore"), quoted by Shawn when he comes before us as Ben, a scientist and a businessman, who is about to read from his memoirs: he awakes on a battlefield, his sword drawn, rabbits leaping and running like horses through grasses of a thousand colours.

This sounds like an authentic dream, no great deduction given that Shawn spends the entire three hours of Andre Gregory's leisurely but compelling production in his elaborate black silk dressing gown. He presents himself, this bald and cuddly gnome, with a permanent frown and a lisping delivery, as a champion swordsman of the bedroom, with or without his women.

As he frankly admits in the funniest passage of his sex-crazed confession: a dick, not a dog, is a man's best friend. For all the women he loves, he's happiest spending time alone with this best friend, and nobody else's, thank you very much: "Two dicks in one house is one too many, and that's the long and the short of it, really, to coin a phrase."

The play is genuinely masturbatory without being offensive. And that's because Shawn takes us on an extraordinary journey of wonder in, and exploration of, his sexual secret life. There's an element of dark fairy tale in his excursion to a castle in the forest where he feasts on mice and is suborned by cats, an echo of the orgies in the novels of the Marquis de Sade, a feeling of inexplicable arrival in forbidden territory similar to those strange sequences in Kubrick's Eyes Wide Shut.

And it's genuinely shocking, and even exhilarating, to see the women in Ben's life play along with the horrible honesty of Shawn's proposition that our sexual relations anyway are much more about those we have with ourselves than with anyone else.

Miranda Richardson's Cerise emerges on video like a troubled, tangled wood sprite, finally assuming the identity of a cat herself, while Jennifer Tilly, his second partner, has the full-blown pneumatic charm of a severely over-ripe plum, or damson in distress.

Ben's third partner is the subdued cat-carer Rose, played with a timorous sensitivity by Emily McDonnell that suggests that she, too, might end up as just one more pussy on the sofa.

Having early on declared that food and sex were the main needs on earth, the first gets swallowed in the pursuit of the second, but there is an assumption that appetite of all kinds can only be satisfied in the comfort of your own dreams, and it's Shawn's need to share these thoughts in his beautifully wrought and highly fraught language that makes his theatre so real and unusual. Plays don't really come into it.

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