Top Dog/Underdog, Royal Court, London

In a room no respectable ghost would dream of haunting, a young black man, fingers trembling, practises the scam card-game of three card monte. Before a wall of filthy, peeling paper and the shadow of something that looks like a dead animal, Booth prattles and shuffles. The door opens and he spins round, gun in hand. Another man enters, in whiteface, wearing a frock coat, false beard and stovepipe hat. He is Booth's brother Lincoln, and the two are locked in another game of deception and death.

Suzan-Lori Parks's play, much praised in New York, is small (one set, two characters), but it takes on huge themes - the subjection of American blacks, their need to imitate or outwit their oppressors, and the dissolution of those efforts in self-destructiveness and loss of identity. But, though Top Dog/Underdog has vitality, it is not ultimately adequate to its ambitions.

Booth and Lincoln ("Daddy told me why we got the names we do... It was his idea of a joke") continue, in their room, the animosities and rivalries of childhood. Lincoln, king of the card-sharps until he became a drunk and his wife kicked him out, enrages his younger brother by refusing to teach him the tricks of the trade. One can understand why Lincoln thinks this would be wasted effort: Booth's big idea for improving his game is to change his name to 3-Card. The depressive to Booth's manic, Lincoln strums his guitar and softly sings, "My luck was bad but now it turned to worse/ Don't call me up a doctor, just call me up a hearse."

Despite his obvious disqualification, Lincoln has a job in which, dressed as his namesake, he is a target in a shooting gallery. But he is worried that he might lose it to a wax effigy. Booth has another bright idea: Lincoln should spice up his act by screaming and jerking about when shot. They rehearse. When Booth shouts "Bang!" Lincoln falls howling, "You mother!" Booth says, "That's good, man."

The director George C Wolfe and Jeffrey Wright (Lincoln) are from the original New York production, which opened two years ago; Mos Def (Booth) joined last year. One cannot fault them, and Def, particularly, is splendid as the loose-limbed simpleton, brimming with unlikely schemes and teaming up with his brother for cod-vaudeville Sambo routines.

But the play is constricted by a lack of discipline and tension. At two-and-a-quarter hours it feels too long, especially as neither the men's lives nor the themes are explored very deeply. In a play that is otherwise realistic, Lincoln's occupation seems to belong in another play, from the sort of symbolic fantasy the Negro Ensemble Company produced in the Sixties and Seventies. And, of course, from the start we know the denouement is limited to Tragic Destiny or Sardonic Irony. Top Dog/Underdog has many good points, but don't go expecting the full monte.

To 30 Aug (020-7565 5000)