Rough Crossings, Lyric Hammersmith, London

Rough Crossings is a gruelling experience but a valuable and fascinating one. It's an ambitious attempt to make a theatre piece based on Simon Schama's brilliant 500-page study of British and American involvement in the slave trade and the story it tells is deeply compelling.

During the American War of Independence, many black slaves fled the plantations and joined forces with the British, who promised them land and freedom in the event of victory. But these men, after having served the Crown, found that the British were not much better than their former masters as promises were dishonoured and they wound up in the freezing wastes of Nova Scotia.

Seeking justice, Thomas Peters (Patrick Robinson), one of their leaders, travelled to London, where he met sympathetic abolitionists who devised a scheme for shipping the former slaves to Africa and liberty in the supposed utopia of Sierra Leone. But Freetown soon became a grim misnomer.

The adaptation is by the novelist Caryl Phillips and, for the first and longer part of the show, the focus flits around too much, as restless as the wooden stage that tilts like a ship's deck. There are some powerful effects in Rupert Goold's production as it moves between continents and tries to keep tabs on wide range of characters. A Handelian anthem merges beautifully with a slave's spiritual; mime and video images projected onto a trampoline-slave ship create the illusion of slaves tossed around by the ferocious ocean. But the wooden dialogue too often fails to bring the individual figures to life.

The drama is much tighter in the second half, which highlights the conflict between the angry black separatism of Peters and the paternalist idealism of John Clarkson (Ed Hughes). With the Sierra Leone Company managing the colony for profit, it's a depressing saga of bad faith, more broken promises, and the rise of racist whites.

The acting is by and large good, and Peter de Jersey brings great sensitivity to the part of the moderate, devoutly Christian David George. But it remains a piece to which you want to give moral brownie points rather than, say, an Olivier. There's bravery and imagination here – and a potential movie, which is surely what it will one day become.

To 13 October (0870 050 0511)