As The Beast Sleeps, Tricycle Theatre, London

Several scenes in Gary Mitchell's As the Beast Sleeps unfold in a grim Punishment Room used by the Ulster Defence Association. A character points out the irony that there were only ever three Catholics in this place – and one of those was brought there not for retribution but an illicit fantasy sex session. It is, accordingly, prime Mitchell territory. In plays like Trust and Force of Change, this Belfast dramatist has concentrated less on the wider sectarian conflict than on the deep internecine divisions within the Protestant community as it struggles to adjust to the implications of the peace process. Receiving its English premiere in a week when the US has declared a war on terrorism, this 1998 drama offers a forceful reminder of the stubborn roots of the tribalisms that terror underwrites.

The focus here is on the uncertain future facing the young men and women who were ardent members of UDA teams. From being treated as heroes for their robberies and other "jobs" and certainly never needing to buy their own drinks, Kyle (Robert Donovan) and Michael Liebmann's volatile Eddie now find themselves persona non grata at the club they used to keep supplied with booze and cigarettes. Mitchell's play skilfully dramatises the kinks and contradictions in the chain of command above them.

Their boss Larry (Simon Wolfe) is rattled at the prospect of becoming a stranded has-been and looks to Derek Halligan's Alec, who has now entered the world of besuited mainstream politics, to take him on board. But Alec's campaign depends on the money he receives from the club run by Larry, so the priority for him is that it should rid itself of trouble-makers. These latter, though, are precisely the young loyalists whom Larry has invested 10 years of his life in indoctrinating and training. How is he to explain to them that changing circumstances have thrown up amended notions of what is meant by such words as "loyalist" and "renegade".

It's as a study of conflicted loyalty that As the Beast Sleeps largely succeeds. Mitchell is adept at fashioning scenes of tense, nudging confrontation, as in the episode where Larry, by twisting definitions and playing on his allegiance to his friends, persuades Kyle to become, utterly against his instincts, the head of a punishment squad. There's an attractively unforced humour in the piece, too, with Dessie Gallagher's highly amusing turn as Norman, the hapless, lanky security officer at the club who can always be relied on to get his limbs broken or to put his foot in it.

For all its virtues, though, I left As the Beast Sleeps nursing a feeling of frustration. The writing adheres so closely to television naturalism that nothing seems to resonate beyond itself or to blossom into metaphor. John Sheehan's production is lucid and absorbing but theatre adds little to this material,which I suspect will be handled more satisfyingly in the forthcoming BBC 2 film version.

To 13 October, 020-7328 1000