Allegiance, Reform Club, London

Alas, Smith and Collins. The Reform Club, on Pall Mall, was where it all began. Gilded pillars, chandeliered ceilings, Biedermeier desks, book-lined walls: 1840s home to Liberal leftists and Gladstonian reformers, like Daniel O'Connell, doughty pioneer of Irish Home Rule.

This sumptuous "set", with subtle low lighting from Nic Ede, could easily have been the library of Churchill's Hyde Park home, where the 47-year-old colonial secretary met in private with the 31-year-old "terrorist" - read patriot - Michael Collins, in a bid to wipe out Dublin's "troubles" and map a future for the Emerald Isle.

Maybe Mary Kenny's psychologically perceptive new Churchill play, premiered by Cryptos, the Reform Club's thriving Military History and Intelligence Society, should be retitled "Seduction". Staged as a one-off in a cut version that works staggeringly well and directed with masterly economy by Brian Gilbert, Allegiance is a witty reimagining of the canny duelling and curt repartee that led Churchill and Collins to metamorphose almost overnight from bitter foes to sparring partners to bosom pals.

Sly, wily, subtle - and cryptic. Kenny's prickly, if uncontentious, drama scores a bull's-eye with Smith's riveting portrayal of Churchill as a master of entrapment. Heinveigles Brendan Coyle's blisteringly abrupt, then cooing Collins with conspiratorial blarney, alcohol and cigars towards conceding a dominion status, shorn of the Protestant north, that Eamon de Valera will spurn. "My political life may be almost over; your great future is before you." But it is Collins who will pay for it with his life.

Smith is a sensational Churchill: this was no puny parody. His and Coyle's perfect pacing mirrored Kenny's finely mapped medley of shifting emotions. Visually, their shifting positions on stage and eloquent body language speak reams. Coyle's Collins gives as good as he gets, yet it is Churchill who makes the running. Smith's puckered lip, fretful strut, pouting pauses, sneaky diversions and sly barbs rooted in military intelligence give way to rheumy-eyed reminiscence and pottering up book-shelf ladders.

"They call you an organising genius. That is why we want you: we want the best," swoons Churchill, mispronouncing "slainte" - "cheers" - surely deliberately. All Britain cheered when they signed the deal. We know what followed.