A star vehicle on cruise control this ain't: Carey Mulligan's Kyra is all composure, a sense of purpose masking layers of hurt, while Bill Nighy's furiously brittle Tom blazes with self-entitlement, ricocheting between bravado and aching neediness. They're so perfectly matched and balanced by helmer Stephen Daldry that watching them lock horns through the first half of this acute revival of David Hare's "Skylight" is simply riveting. But as Hare emerges from behind his characters in the second half, even Daldry's seriously entertaining production cannot disguise the fact that the play's argument is stacked.

Three years after walking out on married restauranteur and wealthy businessman Tom, Kyra is teaching in a tough school in a deprived part of East London and eking out a living a long way across town in a run-down borrowed apartment. A tired evening at home marking books is first wrecked by the unannounced appearance of Tom's 18-year-old son Edward (suitably gangly and impatient Matthew Beard). Why did she leave their home so suddenly? And can she help his father who's rattling around in denial a year after the death of his wife? "He's not what you might call emotionally available," he says.

Answers to Edward's demands are the stuff of the following scenes as, out of the blue and late at night, Tom arrives. A well-dressed emotional mess, he affects casual interest while patrolling her apartment like he owns it. And for the next hour and a half the two of them duck and dive, land blows and tear down each others' defenses as they rake over their past relationship.

Part of the strength of both production and play is the level of often bitter humor coursing through their attempts at truth-telling. Both Mulligan and Nighy never anticipate or indicate the end of a line before it arrives, so the dialogue is peppered with the punch of surprise. Nighy is delicious as he wickedly uses exaggeration for effect. On the page, a line sneering at small-mindedness like "Christ, fucking gardening. If could make it illegal I would" seems near-ridiculous. In Nighy's hands it gets a roar of laughter.

His restless energy is counterbalanced by Mulligan's self-possession. Her calm, low voice is so connected to a startlingly poised physicality that she can hint at her character's end-of-tether exasperation without ever resorting to indicating it.

This being a Hare play, however, their relationship is usefully reflective of wider concerns. Written and set in the early 1990s - but still highly topical - the play is also an investigation of how different people choose to live their lives in an unjust society that is split between the haves and the have-nots. Except that it isn't really an investigation; it's the staunch underlining of a single point of view with superficially winning but less truly convincing counter-arguments.

Kyra's fervently expressed dedication to teaching the under-privileged is supposedly offset by the accusation that for all her idealization of "the people" she cannot love one person. But the behaviors and beliefs thrashed out in Hare's long night's journey towards day make it clear that although Tom has might on his side, Kyra has the moral right.

Loading the dice to underline a particular view in this way is, of course, allowable but it comes at a price. If audiences' sympathies were truly split rather than directed, not being able to make a choice between the two ex-lovers would have a more emotional effect. Coupled with the thirty-five-year age gap between the two actors which makes their potential future relationship seem more paternal than plausible, the tender, extremely well-acted production winds up feeling hugely impressive rather than generating devastating feelings.

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