The Breath of Life, Haymarket Theatre, London

"What the hell are you doing here?" Maggie Smith's Madeleine asks Judi Dench's Frances near the end of The Breath of Life. It's rather late for her to bring this up, as Frances has arrived at dusk and stayed till the following dawn. But, at that point, the question seems an existential one. Why, indeed, are any of us, much less two great actresses, at this vague and arid exercise masquerading as a play?

The casting of David Hare's two-character drama seemed to promise, if nothing else, an entertaining contrast – or contest – of acting styles and personalities. Would Smith swan about, trailing clouds of neurotic grandeur? Or would Dench, less showy but more steely, carry off the honours? Alas, this question remains hypothetical. In Howard Davies's listless staging, which resembles a Zen tennis match, both women are wan and muted. Only a few flashes – the sardonic little lizard flick, for instance, that Smith gives a two-word phrase of apparent sympathy – show us what they might do if better served.

The situation certainly has the potential for fireworks. Frances, a novelist, has called on Madeleine, a curator of Islamic art, to talk about the man they shared: Madeleine's lover was Frances's husband. (If Dench had played the droll, superior mistress and Smith the betrayed wife, the characterisations might have had some piquancy.) But Frances is never angry at Madeleine, only at her former husband, Martin, who some time ago decamped to America with a girl half his age. (Martin sounds no loss, a vain, overbearing chap who calls marriage a "trade union" as smugly as if the phrase were his own and not Shaw's.)

We never, though, get a sense of the time that has elapsed, of how it has changed the two women and their relationship to Martin, how they have discovered new aspects to him, or regarded him from a different point of view. Madeleine's reminiscences are confined to the beginning of the affair, and Dench simply replays the rows she had with Martin, sounding as enthralling as any woman out to demonstrate her moral superiority. Nor is their talk, on the most basic level, plausible: Frances says she wants "the truth" because she is writing her memoirs; Madeleine, at first horrified, starts talking a few minutes later, with no reason for her change of heart.

With all this moaning and musing, an extraordinary amount of information is withheld. We might guess at Madeleine's occupation from the Moroccan furniture, tribal rugs and Syrian screen in her flat, but we're not told it until well into the second of the two acts. It takes the same amount of time to learn that Frances's novels are bestsellers. The needless mystery is irritating rather than intriguing, but it does keep Hare from having to deal fully with the characters. How has Frances's recent success affected this former downtrodden wife? Does Madeleine's love of the East have anything to do with the independence that made her so attractive to Martin? (The Arabs say, "Life is a bridge; on a bridge you build no house.")

When Hare's observations aren't trivial and commonplace, they're blatantly untrue, such as: "The older you get, the easier it is to be happy. It must be nature's way of preparing you for death." An even better way, one feels, is sitting through The Breath of Life.

To 21 December (020-7930 8800). A version of this review appeared in later editions of Wednesday's paper