Gem Of The Ocean, Tricycle Theatre, London <!-- none onestar twostar threestar fourstar fivestar -->

August Wilson died last year, of liver cancer, at the age of 60. Taken before his time, he was at least spared long enough to complete the massive 10-play cycle in which he offers a rich decade-by-decade account of the 20th-century African-American experience.

Gem of the Ocean, the last but one to be written, is the first chapter in the chronological sequence. It receives its British premiere now in a powerfully acted production by Paulette Randall at the Tricycle Theatre, where she has already unveiled three instalments of this mighty epic.

I wish I could say that the latest piece is one of the best of the bunch, but Wilson is not on top form here. In comparison to Joe Turner's Come and Gone, the play which (in its dramatisation of a kind of ritual cleansing) it at times resembles, this is a rather disappointing effort. It exhibits, in a very transparent manner, the author's characteristic unevenness: his superb ear for vivid, tangy vernacular, offset by his penchant for over-insistent, ponderous symbolism; his great talent for creating a deeply humane, humorous and textured social atmosphere, compromised by clumsy, melodramatic plotting.

Like all but one of the plays in the cycle, Gem of the Ocean is set in the run-down Hill District of Pittsburgh, where the author was born and raised. The year is 1904, a time when emancipated African-Americans were discovering that freedom could often feel horribly like the continuation of slavery by other means.

Proceedings unfold in a house that is run as a sanctuary by Aunt Ester, a lady who maintains that she is 285 years old, which argues that she must have been on one of the first slave ships to make it to the Americas. Her visitors include Joseph Marcell's droll, elegantly down-at-heel Solly Two Kings, a scarred veteran who keeps, in place of a watch in his waistcoat pocket, a short length of the chain that once held him captive. He's converted it into an ironic good-luck charm.

Also up from Alabama is the nudgingly named Citizen Barlow, a young man (compellingly played by Kobna Holdbrook-Smith) haunted by the fact that a mill-worker has drowned himself after being wrongly accused of a minor theft that he, Citizen, committed. In the course of the play, this troubled youth is both exorcised and politicised.

The liabilities of supposed liberty are arrestingly highlighted. Anxious about losing cheap labour, white Southerners are, we hear, resorting to violence to stop blacks from migrating north. Not that life is exactly peachy in Pittsburgh; seeking work at the mill, Citizen is offered wages that would scarcely cover the cost of his slum lodgings.

We see how, on the principle of "if you can beat them, join them", Patrick Robinson's unsettling Caesar has turned himself into the very model of a vicious white entrepreneur and ruthless law-enforcer. "If I ever get a plantation, I'd have him to keep the niggers in line," jokes Solly, a man who, having played a courageous and important role in the underground movement that helped slaves escape to Canada, now keeps body and soul together by scraping up dog shit from the pavement and selling it as fertiliser.

But not even a witty portrayal by Carmen Munroe can prevent Aunt Ester from seeming the play's bugbear rather than its star attraction. Relentlessly wise, with a taste for irritatingly oracular statements, such as: "Life is a mystery. It ain't all for you to know," this character is too overtly a symbol of indomitable survival.

And her methods - cleansing Citizen's soul by inducing a ritualistic, trance-like replay of a slave-ship voyage - smack too much of magic and mumbo-jumbo in a play that elsewhere demonstrates the clarity of her political thinking. When Caesar waves the warrant for her arrest, she counters by brandishing another piece of paper (the Bill of Sale that made her someone's property). Natural justice, she intimates, should sometimes override the law. Let's hope that Radio Golf, the last play in the cycle, sees Wilson back at the height of his powers.

To 11 February (020-7328 1000)