I.D. is, as the title suggests, a play about identity, but in the 1960s apartheid South Africa in which it is set, it was also something that was literally carried around: a document to be produced on demand, to prove who you are. As well as naming its owner, the i.d. card also denoted race and sex.

In the published script (Nick Hern Books, £7.99), the South African-born Antony Sher reprints a copy of his own card from that period. "Blanke - White Person" it states in Afrikaans/English, issued in Pretoria on 1.2.65. That's a year before the main event that the first play of this actor-turned-playwright now describes: the assassination of the Prime Minister of eight years standing, Hendrik Verwoerd, the man who was known as the Architect of Apartheid.

Verwoerd (impressively impersonated here by the South African actor Marius Weyers) created the idea of independent homelands, within South Africa, that black people would be sovereign over - leaving the rest of the country to the whites. "These chiefs - they were born here, yet I'll make them aliens," Sher has him saying, and admitting: "Me - the alien, who came here and made himself Chief."

The prime minister was stabbed to death by a temporary parliamentary messenger, Demetrios Tsafendas, inside the parliament building itself, on September 6 1966 - a date whose potentially sinister three 6's don't go unremarked in the play. But then Sher, who has obviously researched his play very thoroughly, doesn't seem to allow any details, however trivial, escape inclusion. "The event was intensely dramatic in every sense", he writes in his introduction to the script, but his play is unfortunately only intermittently so.

The very first scene, introducing Tsafendas in a mental institution and the tapeworm that he claimed lived inside him and might have urged him to do the deed, actually brings the Lintwurm (tapeworm) to immediate theatrical life. As played by Alex Ferns, the worm is an insinuating presence hovering over proceedings and stalking Tsafendas's obviously schizophrenic imagination throughout.

Sher's dense, ambitious play wrestles with fantasy and fact for much of the first act to fill in the blanks on Tsafendas' life. Like Verwoerd, he was also an outsider, born in neighbouring Mozambique of a Greek father and half-caste mother, and who, applying for his own identity card, seeks to be defined as 'coloured' (in the terms of South African racial definitions) because of his love of a coloured woman, Helen.

I.D. also provides Sher with a mammoth part for himself as Tsafendas, which he tackles with his customary full-on assault of fully inhabited and uninhibited character acting. Around him, Nancy Meckler's impressionistic production creates a swirl of scene changes and movement, but it can't rein in an overwritten script that could actually begin at the far more compelling Act Two. It's here that the assassination takes place and the investigation into it follows, as well as the cruel punishment that Tsafendas was subjected to.

- Mark Shenton