Things you shouldn't say past midnight, Soho Theatre, London

In sex, veritas. Straining toward orgasm, the elderly Donald repeats the catchphrase of The Little Engine That Could. His lover, Mark, a therapist, repeats after him, "I know you can." In another part of the split-level set, Nancy, in her moment of ecstasy with Ben, screams, "Do me, you hook-nosed Jew!" The resulting argument propels Nancy, at 3am, to the apartment of her friend Grace, who dispenses comfort, analysis, and advice, as do Gene (Grace's boyfriend), Mark (Grace is his patient), and Donald, on a three-way line and speakerphone.

Peter Ackerman is a New Yorker, as if you had to be told, and he appears to think he has written a comedy about etiquette and self-awareness. What happens, he asks in a preface to the text, when "an inherently good person" – for, in the age of therapy, so are we all – makes a joke or provocative remark, and it "isn't received as he or she intended? What happens when it's not even intended as he or she intended?".

These, however, are only the first, and most innocent, questions stirred up by this crassly commercial play, which not only hits several target audiences (Jews, gays, people in analysis) but takes care to rub them up the right way. Why, one might wonder, is serious, Jewish Ben involved with a silly, perky blonde from the Jew-free (though that never stopped it being anti-Semitic) American South? At the height of their row, he shouts, "Why do you think I go out with you? I don't want to be Jewish!" But, horrified at what he has let slip. Ben changes the subject, and we never hear it again. We know why Grace is with Gene, whom she wants strictly for sex, not dinner and a movie, or even talking – she is turned on by his being a hit man. Indeed, no one in the play expresses horror at Gene's killing people for a living. But then, he's so nice – he even kills in a nice way. "I tie 'em up, plastic bag 'em, leave the room, and five minutes later they're blue. No muss, no fuss."

The real subject of Things is, of course, the way in which our society substitutes fantasy for humanity – other people become no more than a means to getting one's end away. The self-exculpation the group provides for Nancy – which, after all, is what therapy is for – rings as false as the play's ending, a double punch line. One is quite funny (as is much of the play), as well as satisfyingly logical and moral. But the essential dishonesty of the work is emphasised by another character's near simultaneous line, one that traduces character and situation.

Abigail Morris has unfortunately permitted the actress playing Grace to adopt a supposedly comic New York accent that is much too vulgar for the part. But Patrick Baladi is charming as Mark, the therapist with the personality of a time-share salesman, and Vincenzo Nicoli, though occasionally a bit broad, is a delightful Gene. Lucy Akhurst's Southern accent is too blatant at first, and then slips away, but she is so attractive and sweet she almost convinces us there's nothing wrong with being naive. The pleasure of watching these performers, however, cannot take away the fact that Ackerman's play, designed to be a short, pre-dinner conversation piece, leaves a bad taste in the mouth.

To 11 January (020-7478 0100)