The Pillowman, Cottesloe Theatre, NT, London

I was about to say that The Pillowman, the latest pitch-black comedy by Martin McDonagh, would have Kafka spinning in his grave. But since it's a safe bet that Kafka has never stopped spinning since laid there, let's say that this show will have him spinning quite a bit faster. There are affinities with his work both in the nightmarish predicament in which McDonagh's writer-protagonist finds himself and in some of the stories this character has penned. But compared with the moral integrity of Kafka's uncompromising vision, The Pillowman is mere entertainment.

Sure, it raises profound questions. Can art (including McDonagh's own) corrupt and cause damage? Is it parasitic on suffering and does its survival count for more than human life (including the artist's own)? But it toys with those issues, playing fast and loose for the short-term gain of an outraged yelp of laughter here or a shuddering shock there. In the end, you may feel McDonagh has more in common with his pair of creepily teasing interrogators than with his author-surrogate.

Powerfully directed by John Crowley, the play begins in an interrogation room in a totalitarian state. It looks for a moment as if we are in for a rerun of Pinter's plays about torture, such as for One for the Road and The New World Order. Katurian (superbly played by David Tennant) is being grilled about the gruesome content of his parable-like stories. There has been a spate of child murders that seems to have drawn direct inspiration from the grotesque fates that befall Katurian's characters, heedless of the moral framework in which the fictional atrocities occur. It's like a bad dream. "Are you trying to say I shouldn't write stories with child-killings in because in the real world there are child-killings?" he asks the soft cop Tupolksi, who, in Jim Broadbent's lovely performance, is nearer Mr Pooter than Pol Pot. In a totalitarian state, it would, of course, make the perfectly chilling vicious circle if it were the police themselves who were committing the copycat murders. How better to frame a writer suspected of dissidence than by reducing the relationship between art and the world to the crudest model of cause and effect?

The cops aren't smart enough for that, though. The real culprit turns out to be Katurian's brother, Michal. Played by Adam Godley in a manner that evokes the most grating aspects of Michael Crawford and Smike, this sibling is the kind of grown-up infant character whose level of retardedness conveniently fluctuates so that the dramatist can have it every which way with him. Another question the play flirts with is whether it would be kinder to suggest an early suicide to those children who are doomed to lead a miserable adult life. The eponymous Pillowman is a figure in one of the stories who is heavily into kiddie euthanasia. All I can say is that it would have been fine by me if Katurian had decided to stifle Michal a good hour or so before he gets round to it.

Depicted in Hammer horror fashion in dinky overhead sets, the lurid childhood of the brothers suggests a grotesque correlation between success as a writer and proximity to the suffering of others. The parents nurtured the talent of Katurian in one room, while next door they were subjecting Michal to years of untiring torture. This insane favouritism was, it seems, part of an experiment in home-growing an artist. The constant sound of child-torture, you see, made Katurian's stories become darker and darker. Why they couldn't have just faked these grisly sound effects is not clear. There are twists to this sibling story, the playwright playing games with what of truth can be derived from the different adaptations.

McDonagh strikes me a fascinating case of a dramatist with extraordinary technical talent and a disturbingly defective moral sense. I don't like this play, but I can't promise you that I won't go and see it again.

In rep to 27 March, 2004 (020-7452 3009)