As both film-maker and playwright, Neil LaBute has made a career from examining the dubious morals, darker motives and selfish aspirations that lies at the heart of the human psyche, with The Shape of Things (previously premiered at the Almeida) also recently filmed and being shown in this year's London Film Festival.

In his new play The Mercy Seat, again at the Almeida and set in the immediate shadow and aftermath of 9/11, we meet a couple of New Yorkers who are work colleagues and having an affair. It's September 12: the day before, the man was supposed to be in the World Trade Centre at the time of the attack on it, but he missed it because he was having sex with his mistress, who is 12 years his senior, at her loft apartment nearby.

Now his mobile phone keeps ringing insistently, but he refuses to answer it: he's seriously thinking of being counted amongst the missing, and leaving his wife and daughters bereaved instead of dumped for the mistress, so he can begin a new life.

A national tragedy becomes a local opportunity, but there are more than nagging doubts: not just intensely moral ones, but also immensely practical ones - and whether or not both parties are on the same page in their relationship to make all the subterfuge it would entail even worthwhile to contemplate.

As always with LaBute, the stakes are high, but the realisation this time isn't. Faced with a cataclysmic moment of choice that has been thrown at them, LaBute's characters behave as if in an extended therapy session, poring over the minutiae of their relationship with forensic but eventually wearying intensity.

The enormity of the events that have given rise to the situation, however, are barely touched on: "It's beyond belief - biblical" is about as articulate as the characters get. "I mean, those buildings are just, like, gone".

These are people, however, who only see the world in terms of opportunities: when she talks about watching a woman on the street putting up Xeroxes of a picture of a missing young man, and the fact that the whole city is covered in copies, she adds: "Somebody at Kinko's corporate is probably laughing his ass off right now."

Though I was hardly compelled by the play and repelled by these selfish characters, the two actors playing them gave their unsympathetic all in a pair of utterly selfless performances. Sinead Cusack, playing a decade younger than she really is, and John Hannah provide intimate studies of emotional isolation and eventually desolation under Michael Attenborough's direction.

- Mark Shenton