First Night: Fat Pig, Trafalgar Studios, London

Gay-bashing, baby-smothering, disabled taunting, wife cheating: these are the heinous, dark deeds we have come to expect from the characters in the plays of the American misanthrope par excellence, Neil LaBute.

So it's a pleasant surprise to report that, although the playwright has endowed his play with the luridly offensive title Fat Pig, he has written and directed something akin to a rom-com. Albeit a rom-com that comes with a typically bleak LaButian twist of the knife.

This is the UK premiere of the second part of LaBute's trilogy which began with The Shape of Things, in which a man remodels himself to please a woman and ends with Reasons to be Pretty, which has just opened in New York. And the television star-heavy cast on offer at the Trafalgar Studios - Peep Show's Robert Webb, My Family's Kris Marshall and Joanna Page of Gavin and Stacey fame – adds an extra frisson as this is a play which ostensibly deals with contemporary society's obsession with appearances and our prurient interest in the lives of others.

The premise is simple. Tom, a reasonably high-flying Everyman meets a girl, Helen. The only problem is she's fat. Which isn't really a problem, of course, until his bitchy colleagues find out about her ("If she lost, like, 80lb, she'd be stunning" is one typically insensitive comment), and bring Tom face to face with his own ugly vanity.

LaBute has directed a characteristically snappy production, thanks in no small measure to Christopher Oram's simple yet highly effective set made up of a bland but insurmountable revolving wall which slickly dramatises Tom's desire to keep his sweetly blossoming relationship neatly partitioned.

These are, in many ways, stock LaBute characters – the lily-livered, quavering Tom, the priapic office smart-ass Carter and the obsessive, sharp-tongued bitch, Jeannie. But it is the warm and very real relationship between Helen, the "fat pig" (Ella Smith), who uses self-deprecating humour as a defence mechanism but nevertheless lets her defences down, and Tom (an excellent Webb) which distinguishes this from the rest of LaBute's oeuvre.

Unusually for the master of quips, much of the humour comes not from his jokes – though funny, I was hoping for something darker and more robust – but from the excellently directed physical gags of the gifted comedians Webb and Marshall who make a great comic pairing. Page looks uncomfortable and sounds a little too Stacey-like to be believable as brittle bitch Jeannie, but when she finally lets fly at Tom, it's with impressive gusto.

For sure, Fat Pig stretches the not wholly extraordinary notion of an average guy going out with a plus-sized girl a little too thin. And there's a soap opera contrivance to some of the plot – which proceeds in segments with Friends-style surtitles, "That First Meeting With Her" – such as the idea that Tom would introduce a bikini-clad Helen to his work colleagues for the first time. Nor is it much cop as an exploration of weight issues – LaBute handily ignores the fact that fat people can have flaws too. But in the end, it doesn't really matter.

In a much superior second half, when LaBute strips his characters bare, and has Helen heart-breakingly offer to be "stapled" and a craven, yet highly recognisable Tom reveals his soul, the audience can clearly hear a human heart beating under any amount of dramatic flabbiness.