Loyalty, Hampstead Theatre, London

Imagine feeling bitterly opposed to the invasion of Iraq at the same time as being the long-term partner of Jonathan Powell, Tony Blair's Chief of Staff and closest adviser. It sounds like an almost parodically privileged position from which to absorb first-hand material for a play that tightly intertwines the personal and political while also titillating us with its insider-insights and the splatting sound of old scores being settled. But Sarah Helm, a former journalist with The Independent who reported from Baghdad in the mid-1990s, has come up with a debut stage-drama, Loyalty, that stubbornly fails to catch fire, despite its clearly authentic detail and its flashes of bruised comedy.

The author and Powell appear in the piece, which is subtitled a "fictionalised memoir", as Laura and Nick. Set in the couple's Stockwell home in March 2003 during the build-up to the invasion, the first half attempts to pull us into Laura's predicament. Even in the bedroom, her husband is never off the phone to Blair. Their intimacy allows her to listen in with him, notebook at the ready, to conference calls between the PM and global power-brokers – and in a moment that is now tinged with piquant irony, we overhear an impatient, bellicose Rupert Murdoch, in name-dropping cahoots with Donald Rumsfeld, put threatening pressure on Blair. But it galls and distresses her that she is increasingly unsure whether Nick's loyalty to PM and his WMD-justifications for war are a matter of professional duty or brainwashed self-deception.

This central relationship, though, never crackles with sufficient tension and ambiguity. Laura (played by an engaging Maxine Peake, miscast as an incisive intellectual) is saddled with a drama-sappingly unblemished instinct for the truth, while Nick (excellent Lloyd Owen) is obviously decency itself and immune to corruption by the court he serves. Laura thinks that getting things badly wrong is a form of lying to oneself and therefore as culpable as lying. She's vindicated in the second half of the play that moves to a dinner for top intelligence agents in Downing Street on the day the damning Iraq Study Group report is leaked. But Blair, played as a vain, insubstantial poodle in Patrick Baladi's caricature performance, seems to have been pre-judged by hindsight from the outset and Edward Hall's rather lifeless production lacks a requisite sense of hectic media-saturation. On the evidence of Loyalty, there's no need for David Hare to look to his laurels.

To 13 August (020 7722 9301)