Allegiance - Winston Churchill and Michael Collins, Assembly Rooms <!-- none onestar twostar threestar fourstar fivestar -->

With so many puffs in the press about whether Mel Smith, playing Winston Churchill in Allegiance, would light up the politician's trademark cigar on stage, there was a danger that the essence of this new play might disappear in a fug of media smoke. Smoking in any public place is now prohibited in Scotland, and Smith has made much capital out of the fact he could hold but not light a Romeo y Julieta "rolled on the thigh of a Cuban maiden".

Smith is portraying the statesman in the context of a meeting between himself - then Colonial Secretary - and the former Sinn Fein leader, Michael Collins, in Churchill's London home in 1921. With his jowl spilling over his collar, his neck stiffening and his fingers wagging as if pursuing their own agenda, Smith has slipped most convincingly into Churchill's large shoes. He smacks his lips, sometimes swallows his words and punctuates his sentences with a gravelly grunt.

Playing opposite the Greatest Englishman can't be easy but, as the rebel IRA leader with a mythical reputation Michael Fassbender has the unusual advantage of being a direct descendant of Collins. At first both men edge cagily around the other, Collins railing against the Black and Tans - "so named," slips in Churchill smoothly, "purely for sartorial reasons".

There's no denying that Mary Kenny's keenly imagined script gives Churchill the better lines in this sparky encounter between British Imperialism at its most rampant and Irish Nationalism at its most potent. But Fassbender endows Collins with a magnetism and quiet intelligence, his forecasting of his death taking on a real poignancy.

Between them the two characters downed whisky, brandy and champagne in such abundant quantities that surely some Scottish ban on drinking coloured liquid on stage must already be in the pipeline. Whatever unlikely alliance this encounter helped Churchill and Collins to form in the midst of these secret negotations over the Irish question, Smith and Fassbender convey the increasingly warm relationship between the two men, their dialogue only slightly hampered by poor amplification. They wistfully compare family notes, Collins smiling at the memory of the father he worshipped, Churchill blubbing about the death of a young daughter.

But the intrusions on the stage of a narrator in the form of the director Brian Gilbert, who neither looks nor sounds like an actor, nor bears any resemblance to a professional speaker, is inappropriately dressed and clutches a spiral-backed notebook, is a disastrous and unnecessary element in an otherwise absorbing entertainment.

To 13 August (0131-226 2428)