Who, apart from some maker of tasteless musicals, would dare to make a piece of spectacular theatre from the freedom struggle of African-American slaves, who escaped from the plantations to fight for Britain in the American war of Independence and lived to regret it? Caryl Phillips, novelist, playwright and professor of English has had a brave shot at the subject, adapting Simon Schama's book Rough Crossings with the theatre-world's golden boy, Rupert Goold, as instigator of the project and director.

I found the experience fascinating, poignant and illuminating, but flawed by Phillips's methods in fashioning the complex manoeuvre from page to stage. Oddly, Phillips calls himself an "adapter". In reality, though, he attempts something quite different. He has taken a few threads from Schama's 500 pages and woven them into a narrative, both political and personal.

The determined campaign by a few British radicals to terminate the abominable human bodies' trade runs in train with painful vignettes of the slaves' suffering and degradation: exploited by the British, despatched after the war to bleakest Nova Scota and then, in a back to the future move, given the illusory chance of freedom in Sierra Leone, though, of course, with a liberal Englishman in control.

Goold's ingenious staging does not go in for laboured realism. The action, thanks to Laura Hopkins's stylised design, takes place mainly on a mobile platform, which first represents a ship with shackled slaves in violent rebellion against the savage Captain, then the scene of a London musical soiree, where black actors wittily serve as part of the set. This platform even becomes Sierra Leone, a brave new world that comes to grief.

The lines of black-white conflict are neatly articulated through the antagonisms of Patrick Robinson's splendidly defiant, implacable African-American, Thomas Peters, who insists on black self-rule in Sierra Leone and Ed Hughes's too insipid, white liberal Lieutenant John Clarkson, who fights for the slaves to live in Sierra Leone while insisting he should run the show. Phillips rarely manages to encapsulate this ideological essence into tight, powerful dramatic form. He loses sight of the main lines and slips into subplots. He puts length into longwinded scenes of unrelenting verbosity. Goold should have cut and reshaped and made Rough Crossings a better journey.

• Closes 13 October. Information: 08700 500 511.