This astonishing collaborative play, performed by John Kani and Winston Ntshona and co-written with Athol Fugard, is on its third, and reportedly final, visit to London. Far from tiring of it, I find each viewing yields fresh perspectives and proves the work has the inexhaustibility of art rather than the insistence of polemic.

In 1974 one received it almost as documentary: an eye-opening account of the nature of imprisonment on South Africa's Robben Island with its mixture of physical hardship and spiritual resilience. By 2000, when the play returned to the National, we marvelled at the durability of its two performers and the piece's permanent power in a post-apartheid world.

But seeing it again, one becomes more conscious of its ironies and of the way, in showing two inter-dependent prisoners preparing to play Antigone, it explores ideas of brotherhood and freedom and asks if they are automatically synonymous.

A key irony is that John and Winston, as the two prisoners are called, are united by suffering and the bruising intimacy of cell sharing: it is only when John's 10-year sentence is commuted that real divisions appear, leading Winston unforgettably to cry: "You stink of freedom and your stink drives me mad."

But the ironies increase with the climactic performance of Antigone played to the prison authorities: not only does unassailable high art acquire a profound subversiveness but it also lends the mutinous Winston, fearful of being laughed at as the raffia-wigged heroine, an unexpected eloquence and dignity.

Everyone has talked of the play as a testament to the human spirit: less has been written about its theatrical intelligence. However you take it, it belongs to its two performers. John Kani embodies the forceful, domineering partner: Winston Ntshona is forever the harassed victim, who craves peace and quiet.

But, as the play develops, the roles change until the two are shackled together in a three-legged race which remains one of the great images of modern theatre and open, like so much else in this work, to endless possibilities.

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