I.D., Almeida Theatre, London

One man and his tapeworm

It's a double first. I.D. marks the playwriting debut of Antony Sher, redoubtable actor, novelist, painter and illustrator. And it must surely be the first occasion in world drama in which the hero has formed a double act with an intestinal infestation - a tapeworm that emerges personified in the shape of Alex Ferns's louche, shaven-headed sidekick, replete with goggles and a glimmering shit-coloured shirt.

Why is this parasite privileging us with his presence? It's on account of the actions of the main character, Demetrios Tsafendas, a temporary parliamentary messenger who, on 6 September 1966, stabbed to death Hendrik Verwoerd, the Prime Minister of South Africa and one of the main architects of apartheid. Alongside the principled, political reasons he gave for the assassination, Tsafendas claimed that he had been pushed into doing the deed by a talkative giant tapeworm. This allowed the authorities to defuse the political danger of the situation by declaring the assassin mentally unfit for trial and bundling him away to life imprisonment on death row.

In Sher's humane and quick-witted play, though, Tsafendas (played by the author) comes across as a chaotic, generous-spirited misfit, a travel-stained Odysseus who has roamed the world in search of a Penelope and a place he can call home. With a bewildering background of arrests and deportations and mistakenly granted visas, Tsafendas is a man who would promptly reduce any immigration official to nervous collapse. Designated "white" because of his Greek father, he makes a telling mockery of South Africa's brutal racial prejudices by applying to be recategorised as "coloured" on the grounds that his mother was a mulatto. As the title indicates, this is a play about the search for identity - with Verwoerd, who was always conscious of not being born in South Africa, compensating for this by making the indigenous blacks aliens in their own country. In a sense, Tsafendas is the benign reverse image of his victim.

Nancy Meckler's agile and sparely staged production lucidly points up the thematic patterns, while keeping the story - which begins at the end and is presented as a reconstruction - firmly on the move. Particularly strong are the scenes that display the repellent mix of smug cynicism and unsleeping paranoia in the white politicians. There's a mesmerically horrible episode where Verwoerd (Marius Weyers) goes to visit David Pratt, a straitjacketed prisoner who has made a less successful assassination attempt on him. Anticipating violence, the prisoner flinches and hyperventilates as the Prime Minister calmly takes off his jacket and rolls up his sleeves. But a beating would almost be preferable to what he's actually treated to: a caressing account of how Pratt has helpfully proved, by his failed murder bid, the divinely ordained nature of Verwoerd's mission. There's a touch of Pinter in the twisted mental torture depicted here and it's no surprise that soon after this Pratt hangs himself.

The same perverted thinking dictates that Tsafendas, though supposedly mad, must rot in solitary confinement on death row next to the gallows - a hideous visceral closeness to death symbolised here by the way his own cell crashingly mimes the hoist into the air and the fatal drop whenever there's a hanging. As the hymns that attend these executions gradually grow in fervour, the passage of time from 1966 through to Tsafendas's release in 1993 is charted with a moving economy.

Stumbling out into a world where, as Verwoerd's spiteful ghost puts it, South Africa has been "'reclassified', given a change of ID", Tsafendas longs for the luxury he was most cruelly denied in prison: darkness. If Sher's performance sometimes risks turning the character into a sentimentalised plucky little outsider, the play itself is yet more evidence of his vibrant versatility. What will he turn to next - choreography?

To 18 October (020-7359 4404)