Equus, originally performed at the National Theatre in 1973 and subsequently made into a film, is in format the offspring of the whodunnit: what you might call a ‘whydunnit’. The crime, inspired by a newspaper report of the real case of a 17-year-old Suffolk boy, is the savage blinding of six horses. We all know from the start who has done it; the ‘detective’ here is a hospital psychiatrist whose aim is to discover why the boy has done it. The format follows exactly that of a whodunnit, with the gifted detective interrogating the boy and other witnesses until at the end he triumphantly (or in this case, rather less than triumphantly) announces his verdict.

So the success of the production depends enormously on the actor playing the detective part, the psychiatrist Martin Dysart. This was a superbly skilful and convincing performance by Matthew Flexman who took command of every scene in which he appeared, that is to say nearly all of them.

Equally central to the success of the production is the actor playing the delinquent and frequently incoherent boy, Alan Strang, a part which in this production was shared between two young actors; at the performance I saw, it was Tom Fletcher. This is a difficult and inherently unsympathetic role even for an experienced actor, but Tom handled it extremely well, even inspiring our sympathy by the end. Let us hope he stays with us.

There was not a single weak performance from any of the less dominant roles, a tribute to director Steve Thompson’s reputation and his wide range of actor friends. I won’t mention the entire cast, but can’t avoid singling out Julia Riley, who played Dora, Alan’s mother. She was totally believable, and the scene in which she effectively disowns her son was the most powerful and moving in the entire play.

Movement, stage placing, lighting and sound were all as competently and undistractingly excellent as one would expect of this team. I was particularly interested by Steve’s decision on how to depict the horses. In the original production, they were played by partially clad muscular men with highly stylised and artistically impressive horse-like masks. They were frequently the feature most prominently mentioned in reviews of the play, and cynically believed by some to be a significant cause of its box office success. They lent an avant-garde impression to what is actually a rather old-fashioned piece of writing. Steve chose instead to use simple bentwood chairs which the supporting actors lifted above their heads and carried downstage when their presence was needed. His decision was no doubt made on practical and logistical grounds, but it was also a good artistic choice, because anything more elaborate than that tends to distract from the storyline.

A highly skilled production then. What of the play itself? As I have already said, it is in fact a rather old-fashioned piece, full of ‘speeches’ rather than ‘lines’, written more than a dozen years after Pinter and others like him had revolutionised writing for the British stage. The play’s greatest weakness lies in the supposed reason for Alan’s cruel crime, the blinding of the horses. I’d hoped that nearly 50 years after seeing it for the first time, I would at last be able to swallow Shaffer’s proffered solution; I’m sorry to report that I couldn’t.

It’s worth pointing out that Shaffer admitted he had no more knowledge of the real boy who committed the crime than he had read in the newspaper. That boy’s motive was perfectly obvious: he derived sexual pleasure from inflicting pain on others, that is to say he was a sadist. Lacking a willing human victim he was reduced to using animals. Such offences are not particularly unusual in rural areas, and the condition has long been known to psychiatrists as ‘zoosadism’. It has even appeared in popular fiction – Ian Fleming described an example of it in his James Bond novel From Russia with Love, published 16 years before Equus was first performed.

What Shaffer offers us instead is an account of the sexual pleasure Alan gets from riding naked on a horse in the middle of the night, followed by an elaborate sequence of events in which a female fellow stable-hand fails to seduce him in a space adjacent to the horses. This somehow leads, for obscure and vaguely mystical reasons, to the blindings: after all these years still the most monumental non sequitur I have come across in any play. My disbelief having stubbornly refused to be suspended, I left the theatre deploring the waste of so much theatrical talent on such a deeply unconvincing work.