The Mercy Seat, Almeida Theatre, London

"Where were you when you heard about the terrorist attacks on the twin towers, daddy?"

"Well, honey, I was in my mistress's downtown loft, just buttoning up after a blow job." That would be the honest answer for Ben, the thirtysomething New York executive in The Mercy Seat. He's Neil LaBute's new antihero, a man who would very likely have perished in that atrocity, had he not stopped by his lover's pad and accepted a little oral comfort. Not that Ben intends to 'fess up even to having survived. On the principle that it's an ill wind that blows nobody any good, he plans to capitalise on the catastrophe and, taking advantage of his assumed death, head off to a new life with Abby, his mistress, boss and senior by 12 years. The play catches up with them on 12 September, holed up in her apartment near ground zero, psychologically fiddling while New York burns.

LaBute has a good nose for uncomfortable subjects and a compulsion to push his situations to such an extreme that it robs them of real interest. In The Shape of Things the issue of how far it is legitimate for artists to make use of living people was melodramatised, but scarcely illuminated, by the case of an ambitious young installation-maker whose entire relationship with her boyfriend turned out to be a self-interested, cannibalising con trick. It's the same with The Mercy Seat, here receiving its British premiere in Michael Attenborough's almost wastefully well-acted production. LaBute has located a genuine area of unease. As a corrective to the journalism that turned the aftermath of September 11 into a festival of national selflessness and the victims into automatic heroes, his play is right to remind us of the persistence of shabby, exploitative desires and fantasies.

Characteristically, though, it goes too far. Despite the humanising presence of John Hannah in the role, it is hard not to feel that Ben is a spineless creep. Even sexually, his redeeming features would seem to be non-existent. He takes Abby from behind, because he can't look her in the eye, leaving her free to indulge in the guilty fantasy that she is being vengefully rogered by his wife. He even has the gall to give his moral cowardice the veneer of fatherly solicitude. His plan will apparently spare his beloved children the pain of a messy divorce. He is less forthcoming, of course, on the agony they must be going through now as he leaves unanswered the desperate calls to his mobile, or on what kind of paternal devotion could envisage severing ties so brutally and irreversibly.

Able to shift in a moment from searing candour to raw vulnerability, to glower with incredulity one minute and to glow with tenderness the next, Sinead Cusack is magnificent in her efforts to make the implausibilities in the writing look like deep emotional contradictions in the character. There's a superb sequence when she struggles to extricate herself from Ben's embrace and then gives up the fight. Her heart-rending sobs, as they devour each other with kisses, convey both Abby's naked need and her humiliation at being in that hapless position.

LaBute writes in a programme note that his plays have hitherto tended to exist "in a geographic and moral vacuum". If he thinks that he has graduated from that limbo here, he is deluded. There's a perfunctory feel to those moments when he tries to bring the outer horror home to our navel-gazing couple, as in the lifeless little section when Abby goes off to give a possibly widowed neighbour milk for her children. Ironically, given the theme of capitalising on catastrophe, The Mercy Seat does not itself evade the charge of cold-hearted exploitation.

To 6 December (020-7359 4404)