THEATRE / A revenger's tragedy: Paul Taylor on Peter Shaffer's new play The Gift of the Gorgon, at the Barbican

Here's a paradox: a drama about the superiority of forgiveness to blood vengeance that leaves you murderously disposed towards the playwright-protagonist. It's about the only point in Peter Shaffer's The Gift of the Gorgon where your emotions are illuminatingly confused (though it's hard to say how intentional this is).

The new play comes with all the ponderous paraphernalia you expect from this author's work. There's the familiar high-toned but shallow juggling of dualities and the frequent harking back to Ancient Greek legend. Indeed, in a highly original mode of marital spat, the central couple get at one another by scribbling rival versions of the Perseus and Andromeda story, re-angled in a glaringly personal way. These are performed by masked and costumed figures on the large golden ramp that descends like a drawbridge and rests on the hero's pointedly altar-like desk. The symbolism is clunking but apt, since Edward Damson (Michael Pennington), the playwright in question, has a complex of convictions in which the belief that ritual bloodshed 'can clean things' has got mixed up with the idea that theatre needs to get back to its religious (Ancient Greek) roots.

The play is out to test to the utmost liberal enlightened notions about the value of forgiveness. Shaffer systematically trivialises all the issues raised, however, by presenting us with a luridly extreme rather than a hard case. On the subject of hard cases, I'm haunted by a television discussion a few years back in which the parents of children who'd been abducted, raped and murdered met with various professionals. On the one hand, part of you recoiled from the parents' hate-filled, warping obsession with revenge and reflected that they were just as much tragic victims of the atrocities as their dead children. On the other, the sight of Lord Longford sweetly preaching forgiveness as if this were some ethics seminar was almost equally rebarbative. Powerfully conveying the intractabilities of the problem, the discussion was like a first-rate play.

The Gift of the Gorgon can't approach such complexity because its mouthpiece of illiberal recidivism has no personal grief to revenge and (in Michael Pennington's ranting, repellent evocation of this scruffy, self-obsessed windbag) no redeeming feature, unless the character's crusading approach to theatre is to be regarded as one. The hokey structure of the play doesn't help either. The story of the playwright's success, followed by public loathing, his exile on a Greek island and his violent death are told in flashback by his widow (powerful Judi Dench) to the grown-up son he refused to acknowledge (a squandered Jeremy Northam), who is now researching a book on him.

There are lots of unintentionally hilarious pointers to the gruesomeness to come. Take the Act 1 cliffhanger - Michael Pennington has just done a flashback re-enactment of the stamping dance of Clytemnestra he performed fresh out of the shower once in Greece. Then Judi Dench's remembered laughter turns to panicky present tense tears and she brings that session to an abrupt, melodramatic halt. A climactic shower scene that will make the one in Psycho look like a Lux advertisement begins to seem likely.

The switches to excerpts from Damson's archaised, increasingly blood-thirsty plays are handled smoothly in Peter Hall's adroit production. They are hard to watch with a straight face, though. And you can see, from about a mile off, the outcome the drama is portentously plodding towards: that he who keeps the gorgon's head for himself will become the gorgon, and that it's only by creating a real-life scenario of monstrous sick-mindedness that the dramatist will be able to trigger in his wife the vengeful feelings she has resisted on principle. Monumental banality poses as profundity. The assiduously helpful dialogue ('And the light, ah yes, the light . . . so uncompromising. No wonder he loved it') offers patches of illicit pleasure. When his classicist stepmother mentions Aesculapius, the son gushes 'You obviously are very learned' - not enough, though, to challenge her husband's simplistic view that Greek tragedy is a straightforward endorsement of blood-letting.

The Barbican Theatre, London EC2 (071-638 8891)