In Extremis, Shakespeare's Globe, London <!-- none onestar twostar threestar fourstar fivestar -->

Howard Brenton recently described himself as "an old man in a hurry". The Romantic firebrand of Seventies and Eighties theatre - and latterly screenwriter of Spooks - certainly seems to have hit form again after all those puerile anti-government squibs he penned with Tariq Ali.

In the past year, he's produced Paul, a provocatively secular demythologising of the origins of Christianity that nonetheless acknowledges St Paul to be a moral genius whose insights into the nature of love form part of the bedrock of our civilisation. And now at Shakespeare's Globe, he returns to the subject of faith in In Extremis, another drama that benefits from the author's ambivalent feelings.

The real-life fate of Abelard and Heloise furnishes one of the world's great love stories. But their tale has had a raw deal in the theatre, where it is principally associated with the legendary nude scene by Diana Rigg and Keith Michell in a 1970 play by Margaret Thatcher's speech writer, Ronald Millar.

There is no gratuitous titillation in Brenton's astringent and often blackly comic version. A play for today in medieval costume, In Extremis reminds you that Abelard and Heloise were great thinkers as well as lovers, and that their convention-flouting passion can be seen as part of an intellectual project.

Brenton follows the traditional contours of the story - the love (and lust) at first sight between the 12th-century scholar and his 17-year-old pupil; the illicit affair that produces a child and results in Abelard's horrific castration; the ironic aftermath where the defiant sensualists become, respectively, an abbot and a nun who can communicate only through letters.

But, with the world dangerously poised between secularism and religious fundamentalism, Brenton foregrounds the rationalist underpinnings of the couple's dissident stance. Full of rollicking dialectical vigour, the play is, in effect, a battle between the Aristotelian logic of Abelard (the virile, impassioned Oliver Boot) and the fanatical belief in faith as a God-given mystery that drives Jack Laskey's fixated, gaunt and powerfully charismatic Bernard of Clairvaux, a Cistercian abbot who gets high on the mortification of the flesh.

The traditionalist clerics in In Extremis are loath to descend to the "bear pit" of public disputation - the metaphor that pointedly evokes the profane world of Renaissance theatre. Indeed, the play demonstrates that Shakespeare's Globe is a brilliant arena for fiery intellectual debate.

New writing at this address requires an energy and flounce that few contemporary dramatists, confined for too long in studio theatre, can muster. Conquering this space is, one would guess, more akin to conquering the Olivier than any other venue - and Brenton has the knack.

He's by no means starry-eyed about the central couple. Heloise (well played by Sally Bretton, whose very modern looks are apt for such an anachronistically "progressive" heroine) describes herself and Abelard as "philosophical warriors", but Brenton alerts you to the fact that their neglected child is a "war orphan".

There is, too, a slightly off-putting smugness in Abelard's didactic fervour. As in Bloody Poetry, his play about the Romantic poets, Brenton does not airbrush away the collateral damage caused by libertarianism. John Dove's production relishes the argumentative exuberance and the Blackadder-like knockabout.

The attempts to weave the cognate Tristan and Isolde myth into the proceedings feel a bit strained and there are times when the characters seem too knowing about the posthumous power of their story and its long-term cultural significance. But the seductiveness of extremism is vibrantly illustrated by our response to Laskey's vivid, wired-up Bernard. Dominic Dromgoole's new regime has explored fresh possibilities at the Globe and In Extremis brings his first season to a stimulating and satisfying close.

To 7 October (020-7401 9919)