CONSCIENCE, that still, small voice which struggles to be heard in these immoral times, becomes the goading force in Lobby Hero. This fascinating comedy raises questions about truth, morality and loyalty. And sexually motivated power games are played out in the process. The lives of two security guards and two police officers are entwined when a secret is revealed.

The ironically described "hero" of the title is Jeff, a bored, young doorman of a Manhattan apartment block who idles away the empty, small-hours.

But he is soon up against the nagging facts of morality and sexual desire.

Author Kenneth Lonergan whose This is our Youth at the Garrick displays young hedonists casually up to some bad, here shows a twentysomething man struggling to be good.

When Jeff 's security captain, William, arrives on his nocturnal patrol there's more than a touch of David Mamet about the men's vehement verbal dash and swagger. And Lonergan displays a Mamet-like intricacy, too. Black William, played with suitable, dour authority by Gary McDonald, belongs to a world of order, married harmony and conformity. By contrast Jeff, whose un-American relish for irony is nicely pointed in David Tennant's deadpan, delightful performance, has spent his twenties missing out and messing up.

Expelled from the navy for smoking marihuana, poor, girl-hungry and rootless, he has only his charm and intemperate honesty on which to depend.

This honesty is put to the test when William explains his dilemma to Jeff.

The security captain's brother is due to be charged for a murder that he denies committing and has asked William to supply a false alibi. Should the truthful William do so? Jeff, potentially implicated in a cover-up, is faced with his own question of morality.

Bill, a smooth double-dealer of a cop and his smitten Rookie partner, Dawn, arrive on a visit and become caught up in the problem. A subtle, supple dramatic process follows on; a psychological battle of wills and wiles is fought. Since Jeff cannot resist the appeal of Charlotte Randle's impressively acted Dawn, who's tough, flirtatious and juvenile in several fell swoops, a sexual contest is waged between policeman and doorman for the police girl's heart - or body.

Mark Brokaw's beautifully judged production hits all its comic targets, chiefly at the expense of Dominic Rowan's Bill, a chauvinist pig, happy to use intimidation, mendacity and emotional blackmail as weapons in the sex war. Mr Rowan, despite an insecure American accent, incarnates quiet menace.

He looms and glowers, like a bird of prey poised to swoop. But it's Tennant's endearing Jeff, in his battle of conscience, who supplies Lobby Hero with its shots of cerebral excitement.