Theatre: Time is of the essence

There are two things that the chattering classes know about Harold Pinter's 1978 play, Betrayal. One is that it is based on the author's own affair with the journalist and television presenter Joan Bakewell. The other is that it dramatises the story of a wife's affair with her husband's best friend in reverse sequence - beginning two years after the liaison is over (and as the woman's marriage is breaking up) and ending with the party that started it.

Trevor Nunn's revival of the piece at the National, opening next week, provides a good opportunity to range Betrayal against other dramas that employ an artificial time scheme and to argue that Pinter's play finds the best moral and artistic justification for doing so.

When a dramatist allows an audience to view the present circumstances of his characters in the light of their future - a future made literally a forgone conclusion - it can all too often result in a kind of galleried voyeuristic pessimism (it's no wonder that Brecht had little truck with such temporal shufflings). And this may happen despite the playwright's express intentions.

The middle of JB Priestley's Time and the Conways famously flashes forward from 1919 to 1937 and back, from the end of one war to the prelude of another. As well as highlighting the fragility and poignancy of hope, Priestley wanted the structure of the play to reflect his consoling conviction that linear time is only an illusion. Each cross-sectional moment of our lives - including our younger untarnished selves - goes on existing forever in a four-dimensional landscape that we can only see a bit at a time. In practice, though, what makes the impact is the sneak preview of those dwindled lives (glowing socialist Girton girl shrunk to embittered mercenary school mistress) and the way we can trace the origins of such disappointment when the play returns to the earlier period.

A self-professed homage to Priestley, Alan Ayckbourn's 1993 play Time of My Life, has none of his hope but twice his ingenuity, using an elaborate triple-time scheme in the service of a systematic, wilful negativity as it charts the decline of a large family business. All set (rather implausibly) in the same restaurant, its present-tense centrepiece is a fatal birthday party for the overbearing matriarch of this brood. The play cross-cuts between this awkward feast and parallel lunch scenes. The structure permits multiple ironies but the before-and-after scenes just supplement the depressing sense you've had from the start that the need for their mother's approval has ruined her sons' lives. When the father, making a toast at the end, talks of how we are usually too busy worrying about tomorrow or thinking about yesterday to identify moments of positive happiness, his words are undermined by all the yesterdays and tomorrows we have just seen.

The play comes across, in the main, as a cynical stunt. That's not a charge you could lay at the door of Betrayal. Here the structure feels neither in excess of the moral facts nor, though it might be said to constitute an enveloping betrayal, like a mean trick on the characters. Pinter has said that "when I realised the implications of the play, I knew there was only one way to go and that was backwards". Watching or reading it, I have always felt as if I was being steered along a poisoned stream until, in the final moment, the play takes us to its source.

This is the scene at the party where the husband Robert walks in on his wife Emma and Jerry, who has just declared his love for her alone in the bedroom. A vertiginous moment which a (possibly) drunken Jerry tries to cover up with courtly compliments to his best friend about Emma's beauty. Instead of rising to the challenge Robert clasps Jerry's shoulder, and in that gesture of reassuring affection, effectively betrays all three of them. The bond between the men, arguably more important to them than their relationship with Emma; the sense that Robert and Emma's marriage is sustained by the adulterous menage-a-trois - these suggestions crystallise in that second. In the other plays mentioned, the move back in time is a shift from experience to relative innocence. Not so with Betrayal.

Advertised as "looking the way Friends might do if Harold Pinter were on the writing team", David Greig's recent music-theatre piece Timeless took a self-conscious leaf out of Betrayal's book. Focusing on four Scottish twentysomethings, it dramatises their meetings at a favourite cafe-bar. The temporal leap-back is very Betrayal-like: we see the foursome on the brink of an epiphanic experience on a beach at dawn - a timeless moment of pure joy which we already know will haunt them for the rest of their lives.

The point, though, is that in Greig's timeshifts, that moment stands unsullied. I'm not sure that the equivalent memory in Betrayal (of Jerry playfully throwing Robert's little daughter in the air) remains without taint. There's the suspicion, at one stage, that it may have been an attempt to put Robert off the scent by being ostentatiously pally with the kids. Of course, Robert in turn betrays Jerry by coming clean when he finds out about the affair.

The retroverted structure of Betrayal is compelling for many reasons, not just because it allows the audience, with its privileged information, to focus attention on the creepy mechanics of deceit rather than on the convolutions of plot. Sending the characters on a backwards journey that never reaches a state of unambiguous innocence, it leaves them at the point, where, horrifyingly, they will have to start the whole desolating business again. The form of this play powerfully brings to mind two lines of TS Eliot: "After such knowledge what forgiveness?" and "In my beginning is my end".

`Betrayal' is previewing at the National Theatre (0171-452 3000)