Plays like Terry Johnson's Insignificance and Caryl Churchill's Top Girls have famously made theatrical mileage out of putting historical figures into fictional collision with each other. Sebastian Barry now attempts to do the same with a poetically written but dramatically arid new play, Whistling Pysche, that introduces two 19th-century medical reformers to each other on a long, dark night of the soul for them, and an even longer, darker night in the theatre for us.

One is Florence Nightingale - the nursing pioneer and reformer of military hospital sanitation methods, who has been forever immortalised by history. The other is James Miranda Barry - an army surgeon who also did pioneering medical work, but is nowadays remembered more notoriously for a secret revealed after his death.

The fact that he is played by Kathryn Hunter in Robert Delamere's premiere production of the play is part of that secret. But Hunter is always such a powerfully strange but compelling actress that her presence here doesn't so much solve the mystery as add to it. Who is this diminutive creature scuttling about in army uniform, remembering his poodles (whom he called Psyche, hence the title of the play), and a career that took him from Cork in Ireland, where he was born, to British imperialist outposts from South Africa to Canada?

We have ample opportunity to find out in the long, discursive monologues that Barry supplies him with. Complete with a convincingly balding pate and stooped gait, it's another in Hunter's marvellously inhabited creatures. But there's no drama. And nor does it ignite in the portrait of the more staid, severe figure of Nightingale - taken by Claire Bloom in a no less total transformation of this usually beautiful actress.

Set in the waiting room of a Victorian railway station - atmospherically conjured in Simon Higlett's design and Tim Mitchell's evocative lighting, with an air of dislocation underlined by a rolling grey film on a rear projection by Jon Driscoll and the insistent musical underscoring of Ross Lorraine - they are awaiting some kind of redemption out of what may be purgatory. But though there are shafts of brilliant writing throughout, prose alone doesn't make drama, and I, too, longed to escape this particular theatrical purgatory.

- Mark Shenton