Three years after Blasted, Sarah Kane continued her theatrical purging with Cleansed (1998), a catalogue of violence, dead rats and humiliation so extreme that usual critical terms of dislike, enjoyment, dismay and admiration are not only superfluous but irrelevant.

Kane subjects actors and audience alike to a numbing agenda of enforced participation in an Orwellian institution of - what, exactly? - torture, correction, separation and sexual coercion supervised by a janitor, or schoolmaster, called Tinker.

Jack Tinker was one of the critics who blasted Blasted and even called publicly for the Royal Court to be stripped of its funding for presenting it (he later recanted). Kane's riposte, if that is what Cleansed was, redoubled its fury and poetic wonder at what we are capable of doing to each other, and like the first play, was seen at the time as a postscript to the wars and atrocities in Bosnia ("cleansing" is more than merely ethnic).

Katie Mitchell's remorseless, relentless production, which contains a performance of harrowing innocence and (literally) stark naked shell-shock by Michelle Terry, is set in a nightmare place of correction, designed by Alex Eales, with deafening alarm bells, chicken wire fencing, peeling walls and a pitiful array of sun-flowers peeking through the floor boards.

Terry's amazing Grace is like Viola after the storm - and the earthquake, and the massacre - separated from her brother Graham (Graham Butler), exchanging clothes after Tinker (Tom Mothersdale) has syringed his eyes. Two other blindfolded prisoners, a gay couple, Carl (Peter Hobday) and Rod (George Taylor), have their devotion tested by amputation and other horrors. Matthew Tennyson plays another inmate.

Tinker has a trio of black-masked assistants who push people around but he escapes himself to another location, a strip joint, where he pleasures himself (yes, he does) while a "woman" (Natalie Klamar) gyrates provocatively in a see-through glass chamber/shower before stepping out and entering the orgiastic fray herself. How we treat each other is a result of how we see each other, and right on cue Blondie comes up on the soundtrack: "All I want is a vision of you."

Tinker wants the woman, Grace wants her brother, Carl wants Rod, but, as Mick Jagger sang, you can't always get what you want. In Grace's case, she ends up as a filling in a sex sandwich and seems utterly depressed by having a penis transplant. Suzan Sylvester was pretty remarkable as Grace in the first production, but how Terry gets through this show and does what she does so beautifully, I have absolutely not a clue.

I don't think the play is disgusting, any more than certain great Jacobean tragedies are disgusting, or King Lear is disgusting. Kane was never in the business of giving you "a good night out," anyway. She set out to shock and anaesthetize her audience with their own inherent wickedness and humanity. And Cleansed is Kane at her most utterly uncompromising.

Cleansed runs at the National Theatre until 5 May.