The Observer, National Theatre, London

A few votes short of a landslide

She is the observed of all observers, and a good woman in West Africa: Fiona Russell, ebulliently played by Anna Chancellor, is deputy chief of the mission to scrutinise a fair election in a fictional country where they speak the Nigerian language of Igbo and an unpopular, repressive President is expected to be returned.

Keeping an eye on Fiona herself is a crumpled, comically inflected British civil servant (James Fleet) who is monitoring Fiona's emails home while she is supervising a small observation team, liaising with a cynical BBC reporter (Lloyd Hutchinson) who is stuck on the never-ending treadmill of touring democracy – "We can't all be John Simpson..." – and developing a personal relationship with her translator Daniel (Chuk Iwuji).

It is an immensely promising scenario, scrupulously monitored by the director Richard Eyre, but Matt Charman's play in the National's Cottesloe auditorium never really catches fire. The excitement surrounding the ballot box is insufficiently contagious until Fiona is summoned before a central electoral committee who make it clear what they think of her "flatpack" of political ideals.

Fiona believes that she must try and get more voters to take part in the second ballot, not necessarily because she wants them to oust the President, but because it makes for a fairer process. But as these two things amount to the same goal, she becomes embroiled in accusations of impartiality and political intervention.

The same could be said of the Americans' invasion of Iraq, or Tony Blair's commitment to regime change, or the West's attitude to a dictator like Robert Mugabe. How far does our sense of justice extend to looking exactly like that in other peoples' back yards? In the second act, Fiona visits a village in her campaign to register more voters and discovers first-hand intimidation, laissez-faire and indifference.

Charman, a talented playwright whose previous subjects include the culture of dog racing and polygamy in south London, paints on a broad canvas. And Eyre's production gives him access to a large paint box.

The stage is hung with brown raffia screens that disappear to reveal a polling station in a rain-drenched school room of corrugated iron and wooden desks, or a parched landscape in the heart of the country. And when Fiona steps into the screen at her own final press conference, you get a real sense of the momentous implications of putting a do-gooding conscience into practical effect.

Rob Howell's design and Neil Austin's lighting certainly convince you that they know the terrain, and they also allow for the slick injection of projected news reports; a tickertape of the countdown to polling day runs across the top of the stage with a portentous musical accompaniment by Richard Hartley. The play is dotted with bright cameos, from Cyril Nri as a bigwig and a chaotic bartender, Joy Richardson as an irate mother and Leo Bill as an impetuous member of Fiona's team.

To 8 July (020 7452 3000; www.nationaltheatre.org.uk)