Theatre NOW YOU KNOW Hampstead Theatre, London

The themes at the heart of Michael Frayn's latest play are secrecy and revelation - how the one can be the lubricant that enables us to rub along together, and how the other can be a potent destructive force. Now You Know is set in the offices of a campaign for freedom of information, all of whose staff have their own secrets, their own little worlds: there's Kevin in the mail-room, with the bag that is never opened, Liz, giggling behind her hand at her own private jokes, and Terry, the campaign's charismatic director, whose upfront, open manner hides some dark corners.

The play charts the disintegration of this cosy universe after the arrival of Hilary, an attractive, successful young civil servant who has fallen for Terry and his well-rehearsed spiel on openness: as more and more secrets come into the open, everything falls apart.

The one thing you don't know by the end is how good a script this is. It undeniably has flaws. In adapting his 1992 novel for the stage, Frayn has compressed the action, creating at times a sense of unseemly haste - while Terry is talking about how slow off the mark he's been to realise that Hilary is after his body, you're still gasping at what a fast worker he is. And the final collapse of the campaign feels too schematic, not something that sprouts naturally from what's gone before. As you'd expect from Frayn, there are plenty of shrewd ideas, tied to some effective comedy. It's a mark of his skill, and humanity, that he can make comic figures out of the disabled Kevin and the young, black, loutish Kent, his mail- room colleague, without seeming to patronise - showing you how the comedy arises from the roles they're trapped into by other people's prejudices, how the secrecy of their lives is foisted on them.

But Michael Blakemore's production too often feels like a sketch rather than a finished product, so that while you can see where Frayn is going, it's difficult to be sure that he's got there. The script is elliptical and allusive. It leaves it to the actors to convey what's going on through looks and smiles, nods and winks - too often the message doesn't get across. And the pace constantly founders around Adam Faith, beautifully miscast as Terry. While everybody else in the office has their little secrets, it's essential that Terry should be big, blustering, rhetorical, determined to show that he has nothing to hide; whereas Faith inevitably projects inward canniness and discreet charm - the wrong qualities for the part.

The production is skewed by Rosalind Ayres's excellence as Jacqui, Terry's office manager and part-time mistress, horribly gooey as she talks about Poops and Pippi back home in Sunningdale, or projecting a wonderful malevolence as she rattles out venomous endearments ("My honey, my precious"). And while Louise Lombard is also miscast as Hilary - too pretty, too smart - she conveys well the passion and indignation burning under her outward reserve. This isn't Frayn at his very best, but it's still a thoughtful, witty piece; and you worry that, buried under a production that isn't nearly slick or sharp enough, it may just remain a well-kept secret.

nUntil 26 Aug (Booking: 0171-722 9301)