Lobby Hero, Donmar Warehouse, London

Our theatre has lately been rich in idiots, to whom Kenneth Lonergan's Jeff is a worthy addition. Jeff sits behind the security desk of a Manhattan apartment block, reading and sleeping his way through the night shift and desperate for company. Like anyone in such circumstances, Jeff, when he does have a visitor, talks too much and says the wrong things, but one feels that, even among friends and fresh as a daisy, he would be the same. When the other characters in Lobby Hero are at the height of the worst crisis of their lives, Jeff thinks that it's just the right moment for a friendly little joke. And, for someone who shuns reality, Jeff is spectacularly inept at making things up: He thinks he can fool a policewoman by telling her about "a friend'' who is in a "hypothetical'' situation exactly like a current case.

When his boss, William, blows up at his insensitivity, Jeff says: "This kind of problem is not exactly within my forte.'' "Which is what?'' demands William. Jeff cannot answer this.

William's kind of problem, though, would be a tough one for anybody. His brother, charged with rape and murder, has no alibi and wants William to provide one. William has clung to respectability as tenaciously as his brother has to drugs, but family ties and fear that his brother – who may be innocent – will be railroaded by an inept and prejudiced system (he is black) make him decide on perjury. Jeff, who knows William has lied to the police, is torn by self-interest, loyalty, a social conscience and the desire to impress Dawn, the local policewoman. "If I was going out with a cop,'' says Jeff, "I'd feel somewhat safe.'' Dawn's police partner, Bill, is also after her, but, though vicious and a practised liar, he is no match for Jeff's stupidity. Lobby Hero has a great many more laughs than one would expect from a play about a moral dilemma – more even than most comedies. But, though consistently entertaining, it is ultimately unsatisfactory. The plot and ambience are familiar from television, and the arguments about issues would be marked "could do better'' if made by a sixth-form debating society. William's confession to Jeff and the means by which Bill's deceit is exposed beggar belief, and the play stops, rather than ends, in a smudge of sentimentality.

On balance, though, I'd recommend Lobby Hero for its well-drawn characters and for Charlotte Randle. Anyone can see how good she is, but only a New Yorker can know that she's perfect. Dawn's tough mannerisms that only emphasise her schoolgirl naïveté, her mixed motives of idealism and self-justification, her confused rage and pain at being betrayed – Randle beautifully brings out all those and underlines them with an impeccable lower-middle-class Long Island accent. David Tennant also paces and colours the hapless Jeff with great skill, but Dominic Rowan, as Bill, seems to be acting in a David Mamet play, giving us a comedy routine rather than a character. The part of William has the least scope, but Gary McDonald ruins it with an accent that makes him sound not uneducated but insincere.

To 4 May (020-7369 1732)