What a beguiling sense of surprise and novelty Harold Pinter's The Hothouse engenders in a powerfully acted production by Ian Rickson that bristles with unease and menace. Written in 1958 and never performed until 1980, this politically nuanced black comedy and surreal fantasy takes place in some variety of mental hospital. It belongs in a Pinter world of its own, one which I had never experienced before.

You recognise the quizzical aggression, terseness, and unpredictability of the way the staff talk. The diction of these bureaucratic fixers who make a wicked mess of running the hospital sounds as familiar and as Pinteresque as you could wish. There are even distinct echoes of incidents in The Birthday Party and The Caretaker. The subject matter of The Hothouse, though, marks a radical departure. It strides into the public world, away from the private/personal dramas of family, sexual and domestic concerns in which Pinter specialised for most of his extraordinary theatrical career.

Imbued with the spirit of Franz Kafka, who envisaged the state's long arm and its judicial system as secret agencies from which rationality had long since departed, Pinter's Hothouse shows how the coercive machinery of state oppression works, forcing entry into people's minds and trying to change them. The scene in which Leo Bill's ambitious new staff member suffers electrodes round his wrists, excruciating sound-blasts in his ears and demented questioning with fanatical forbearance, illustrates the process to shudder-making effect.

Hildegard Bechtler has dreamed up an absolutely superlative stage design that envisages the hospital as a great, dilapidated hulk, supported by girders and equipped with rickety Fifties office furniture. It dips down into a white-tiled stairwell and up to a wrecked, windowless, sound-proofed room where behaviourist experiments happen. The cat-and-mouse action centres on the discovery by the asylum's director, Stephen Moore's magnificent, choleric Colonel Roote, that one patient, known like all the rest by a number rather than name, has had the impertinence to die. Worse, a woman has given birth. How could such things have happened in this world of order and rotas, he wonders. A bizarre whodunnit investigation begins, together with an inquiry to discover the impregnator.

Finbar Lynch's smooth, dehumanised Gibb, who fills in the gaps in the colonel's misty or dissembling mind, finds the colonel's mistress Miss Cutts, Lia Williams, smitten by his line in psychological torture. He reacts with typical, lofty dispassion. Almost every cruel or corrupt action, every power game played by Roote with Gibbs and Paul Ritter's scheming Lush, is despatched with unflinching decorum and in rare, black comic style. How fitting it is in Pinter's enthralling nightmare world that the inmates' murderous revolt changes nothing.