Edinburgh Fringe: The Fever, Traverse Theatre Clare Coulter talks for 90 minutes on a black stage and makes you feel both impressed and ashamed. And all without raising her voice. By Adrian Turpin

If there's been a better production on this year's Fringe than Wallace Shawn's The Fever, performed by the Canadian Clare Coulter, then I'll eat my copy of the Communist Manifesto.

A silver-haired, middle-aged woman, dressed entirely in black, sits on a black chair in the middle of a bare black stage, and talks without interruption for 90 minutes. She carries with her a glass of water and a piece of paper as she enters, but neither reads nor drinks. Coulter rarely raises her voice, and yet no one could accuse her delivery of being monotone or soporific. Instead it's full of subtle inflections, the soft falls and rises of genteel East Coast America, which both mask and highlight the terrible nature of what Shawn's play has to say.

In a poverty-stricken country teeming with revolution, Coulter's character starts by describing how she wakes shivering in a hotel room, and crawls to the bathroom. She has a fever. But the story she tells is of a different kind of malady, namely the numbness that overwhelms her as she realises the inequalities of wealth between poor and rich, the Third World and the First. "The life I live," she says, "is irredeemably corrupt, it has no justification." Every moment of luxury and comfort is only possible by a wilful blindness to the fact that it is purchased at somebody else's expense.

The Fever is, in many ways, a deeply unfashionable piece. It's a sparkling Marxist-inspired polemic (the woman recalls opening her door one morning to find a copy of Das Kapital wrapped in brown paper on her doorstep). It works by continually implicating the audience in a silent crime. When Coulter uses the word "you", it's not addressed directly at the audience but to some imagined companion or to herself. But Shawn knows that his audience is likely to be middle-class and liberal. He knows their weak spots, so that each "you" rings like an accusation: "Have you ever had any poor friends?"; "Your life is another example of getting away with something".

When Coulter does stop talking - for 10, maybe 20 seconds - the silence is heavy with guilt: strangely, it feels more like being in church than in a theatre. Yet the play never leaves one with the sense of being hectored. Shawn's writing is too unshowy for that, and the character he has created is too full of inner conflict. "I know what I like," she says: warmth, cosiness, pleasure, love, mail, Matisse paintings, beauty. Her political fervour is punctuated by sensuous recollections of parties and balloons and delicately wrapped children's gifts.

She's at once thrilled and ashamed, just as The Fever leaves you thrilled and ashamed to be sitting in a snug theatre on a cold evening in a prosperous district of a western European city.

Traverse Theatre, Cambridge St. Final performance 2.15pm today (0131-228 1404)