The Island, Old Vic, London

John (John Kani), going over a play the two prisoners are going to perform, says in exasperation to Winston (Winston Ntshona), who keeps failing to take it in, "I'm warning you: I'm doing this plot for the last time!" Well, thinks the viewer, if he doesn't know it by now, he never will. For the actors have been doing The Island since 1973, when they and Athol Fugard (forbidden, under the apartheid laws, from collaborating with blacks) created in secret this drama about the maximum-security prison that for nearly three decades held the future president of South Africa.

The play begins with a 15-minute mime of the prisoners' forced labour. Those for whom, as it is for me, that medium is one of the worst four-letter words may be assured that there is nothing remotely soulful about John and Winston's work. They dig; they fill and push wheelbarrows, grunting beneath the glare of the overseer; they run a ghastly three-legged race, shackled at the ankle. These actions are even more painful in light of the actors' present age: Both are bald, Winston shrivelled, John fighting his loose flesh as well as time and the whip.

The day over, the men retreat to their cell, a small platform on the bare stage. There is much odd-couple comedy as the buoyant John tries to make the crabbed, tetchy Winston enthusiastic about his role at the forthcoming concert party. One can understand Winston's reluctance, since the part is Antigone, but, strangely, he doesn't worry about how it will be received by the prison administration; what troubles Winston is being ridiculed for appearing in a false bosom and a string wig.

The Island consists, a bit too neatly, of four sections: the mime, the comedy, an agonising passage of yearning for home, which encapsulates the men's dilemma (to survive, they must deny some of their humanity; to remain human, they must feel some pain) and the play-within-the-play.

Despite the grandeur and the beauty of the play, it comes across as a bit preachy – or at least unrealistic. How has Winston, so bad at learning his lines, managed, with only a day's rehearsal, to be calm and word-perfect? Wouldn't the tension be ratcheted up if he had some trouble getting the words out – a hesitancy that might convey his nervousness at confronting his masters with a play about the cruelty of the law?

Still, there is no denying the magnificence of his gesture (and, indeed, that of both actors' performances) when, on his last lines, Winston removes the wig and padding that identify him as Antigone and speaks as himself about death and honour.

With the passing of apartheid, The Island remains powerful enough to stand as a generic drama of injustice. But it is a more poignant one than it was 30 years ago, when the enemy was obvious and the cure seemed simple. Now that John and Winston's struggle has attained its end, evil has been replaced with horror – blacks killing whites and one another, and killing themselves with Aids. John speaks more truly than he knows when, playing a weatherman to entertain his cellmate, he says, "Conditions locally remain unchanged... It's cold, but the worst is still coming."

To 13 April (020-7369 1722)