Under The Black Flag, Shakespeare's Globe, London <!-- none onestar twostar threestar fourstar fivestar -->

Silver's original pirate material
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Long John Silver is so well-defined a figure that you feel he must have emerged from the womb fully equipped with his crutch, his parrot and his catchphrase "Ah, Jim lad". But Simon Bent imagines the early (two-legged) life and adventures of the world's most famous pirate in Under the Black Flag, the first new play at Shakespeare's Globe during Dominic Dromgoole's inaugural season. The CV he invents for the future old rascal includes, you may be surprised to learn, a subscription to the Leveller philosophy and a conversion to Islam.

Ranging from the England of Oliver Cromwell to the Barbary Coast and the pirate republic of Rabat-Salé, this populous historical drama piece is something of a departure for Bent, who has tended to write plays about contemporary losers, often set in confined spaces in west London and influenced by Pinter and Joe Orton - if sometimes invoking the spirit of Chekhov. The last place you expected to find him was on the high seas in the 17th century.

It turns out that writing screenplays has whetted Bent's appetite for working on a larger scale and that the Long John Silver project began as a (rejected) film treatment. The play he has fashioned from the material doesn't always keep a shapely control of the life swarming round this broad canvas and there's a streak of political correctness in the piece that feels at odds with the prevailing jokey-serious tone. But it makes for a fitfully entertaining and impressive show.

The play begins with the execution of Charles I and public rejoicing at the country's new-found freedom. The suspicion quickly grows, though, that, in choosing Cromwell, the English have swapped one oppressor for another. Neglectful of his wife and daughter, the young John Silver (the insufficiently charismatic Cal MacAninch) is working as part of a con-team with his father who pretends to be a holy fool sent by God in search of saints and preaching the dissident message that "Sin is the straightest, most true path to God". It's their bad luck to cross paths with the Lord Protector and his Roundheads, who (to droll music by Orlando Gough and with lyrics that nod at Gilbert and Sullivan) sing of how "We are the major model/Of a modern moral army". The father is shot dead and Silver is press-ganged into the navy by the ruthless zealot Mission (Robin Soans).

The ship is seized by pirates, led by Kees de Keyser (excellent Nicolas Tennant), the self-styled "King of the Barbary Coast", and Silver and his friend are to be sold as slaves. Bent has unearthed some fascinating historical data - for example, he's discovered that Hamlet was adapted and staged by slave traders so that potential purchasers could make bids for the performers while watching the play. He makes use of this bizarre fact in a rather confusing flesh market scene, where a slave insurrection taking place under cover of Hamlet is quelled. The play is intrigued by the idea that the multi-national group of pirates who operated from Rabat -Salé had a parliament that was a like a more egalitarian parody of the one in England. It's here that Silver's Levelling views ("If it's equal shares in the risk... then it should be equal shares in the prize") win over his comrades and excite the undying hatred of De Keyser.

Making not-so-veiled allusions to the present "war on terror", Under the Black Flag pointedly presents the Roundheads as the fundamentalist fanatics in the face of the less extreme Muslims. Silver emerges as a man whose sincere desire to do good is foiled at every turn, leaving him in the end as the personification of an apocalyptic impulse to vengeance - a figure it's hard to see developing into Stevenson's frightening but oddly attractive villain.

As a play for today in 17th-century dress, Under the Black Flag is good but too pantomimic, and nowhere near as sharp as 5/11, Edward Kemp's provocative recent piece that drew implicit parallels between the Gunpowder Plotters and al-Qa'ida. It's not helped by a splendidly cast, yet strangely under-energised production by Roxana Silbert. This never animates the play's nautical aspect to the atmospheric degree achieved here last year in the stagings of Shakespeare's late romances.

To 12 August (020-7401 9919)