THEATRE The Censor Royal Court, London

With just a desk separating them, a young woman challenges a man in authority and the effect on him is devastating. It's a classic situation. We could be talking about the seismic encounters between Isabella and Angelo in Measure for Measure, or about the catastrophic collision of professor and PC-crazed student in Oleanna. Anthony Neilson's The Censor - deservedly revived now for a further run at the Royal Court's Theatre Upstairs - gives this set-up several unsettling twists. Not least of the puzzles in this powerful, deeply enigmatic work is the precise function of the woman who comes across as both wrecker and redeemer, at once a healing projection of the man's own "anima" and a femme fatale sent to him by some inscrutable fate.

Intense, austere, witty and poignant, the author's production beautifully communicates the play's elusive overlappings of reality and unreality and the stealthy, simultaneous progression of contradictory meanings. Down in the noir-ish nether world of his basement office, Alastair Galbraith's excellently uptight, skinny, Scots censor spends his life wielding scissors over hard-core porn films. Enter Miss Fontaine, a darkly attractive, disconcertingly self-possessed director (superb Jan Pearson) who wants to convince him that her movie, which consists of nothing but graphic sex acts, is, if he could only see beyond the images, a subtle history of a human relationship.

Hers is a philosophical crusade. The joke is that, to further it, she is intent on adopting the missionary position in more ways than one. Pretty soon, cool as you please, she's got her hands down the reluctant censor's trousers and, from what she finds - or rather doesn't find - there, is making deductions about his sexual insecurities and his marital misery. We get periodic glimpses of the latter on the occasions when his wife (Alison Newman), driven to infidelity by his hang-ups and his silence, materialises at the side and old conversations are replayed even as Miss Fontaine is sussing out, and then therapeutically satisfying, the coprophiliac voyeurism which turns out to be at the bottom, so to speak, of the censor's sexually paralysing shame.

I do not, as some have done, see the play as too weighted in favour of Miss Fontaine and her philosophy. True, her implacably determined tolerance does awaken love in the censor; acts do, in this case, lead to deeper meanings. And, with Miss Fontaine promptly getting herself killed, it might even remind you of Shadowlands - the censor, like the repressed CS Lewis, belatedly stirred into feeling by a free-spirit and then left cruelly bereaved.

But Neilson's play is potent because its female healer remains an ambiguous figure. There's something more than faintly mad about a woman who thinks you can deduce entire Who's Who entries from watching people have sex. "Could you tell, for instance, that the man's previous girlfriend was Asian? That the woman was brought up in care?" she demands of the distraught censor. And her manner of leaving - not without love, but with a sort of eerie impersonality - made me think, weirdly enough, of Mary Poppins, another mysterious agent who clears up dysfunction but can't allow her subjects to get too close.

Reduced to shuddering, misunderstood tears by her death, the censor is last seen re-viewing her film. The recorded voice of the young Barbara Cook swells through the theatre in a version of "Till There Was You" that is the last word in fresh, ardent gratitude for a life-changing love. Never can it have been played in a more equivocal context. Highly recommended.

Royal Court Upstairs (at the Ambassadors), West Street, London WC2 (0171- 565 5000) Paul Taylor