It was always clear that the ever-reliable Simon Callow would rise to the demands of his star role in Peter Shaffer's Equus. He brings all the necessary gravitas - plus a welcome measure of wit - to his portrayal of the committed, if a tad world-weary, shrink who is probing the disturbed mind of a 17-year-boy responsible for the blinding of six horses in a horrifying attack with a metal spike.

No, the revelation in this new touring production - a recast version of last year's West End hit with Richard Griffiths and Daniel Radcliffe - is the performance of Alfie Allen as the tortured teenager. It would be natural to assume that this tyro thesp - the son of actor Keith Allen and younger brother of the more famous (thus far) singer Lily Allen - got the job on account of who he is. Had this been the case one might have called his recruitment a rare instance of nepotism being given a good name. In fact, Shaffer himself was responsible for his being offered the part, having been impressed by his work (his only substantial work to date) in the film Atonement.

The writer's championing of him proves to be entirely deserved, for the young actor turns in a riveting, powerfully spoken performance that is frightening and pitiable by turns. I can pay no greater compliment than to say he almost effaces the memory of Peter Firth's Alan Strang in the original 1973 National Theatre production. He certainly packs more punch - a question of size, perhaps - than eight-stone Daniel Radcliffe managed, excellent though he was.

Allen beautifully presents the pent-up emotions felt by the boy. These arise from the impact on him of the deep religiosity of his hellfire-spouting mother (Helen Anderson) becoming confusingly overlaid in his mind with images of horses and memories of the excitement stirred by childhood encounters with them. A key moment comes when his grumpy atheist dad (Colin Hurley) replaces a Crucifixion scene on the boy's bedroom wall with an iconic portrait of a horse. The result for Strang is a new religion all of his own, with 'equus' the object of worship.

To a degree, shrink Dysart, who is sadly lacking passion in his own life, is jealous of the boy's wild ecstasy. (How we laugh when we hear of the knitting - Callow brings a memorable emphasis to the word - that is Mrs D's chief enthusiasm.) But, as Linda Thorson's kindly magistrate properly points out to the doctor, the boy is in pain and has a right to expect help.

I have seen the play a number of times in the 35 years since it was written. I confess to now finding it - as I found it last year in London - rather too pat in its unravelling of the problem, too urgent in its pushing of the now discredited theories of the psychiatric guru R.D.Laing. But it remains compelling theatre all the same.

Equus is at Milton Keynes Theatre until tomorrow. Box office: 0870 060 6652 (www.miltonkeynestheatre.com).