It’s astonishing that there haven’t been more plays on the vexed topics of post-colonial guilt, British (non) intervention in contemporary African politics and the inability of international agencies to act when they spot foul play. Credit, then, to Matt Charman, for moulding these issues into a gripping and powerful work, although far less credit for his final dramatic conflict resolution skills.

In a fictitious former colony in West Africa, a group of international observers work to ensure that the forthcoming presidential elections are those two buzzwords “free and fair”. Fiona Russell (Anna Chancellor), the frazzled deputy chief who has been unfairly overlooked for the top job, supervises efforts on the ground.

When the President unexpectedly fails to secure a majority in the first round of polling, Fiona oversteps the remit of her job to embark upon a huge voter registration drive, to the increasing disquiet of her local translator (Chuk Iwuji).

The parallels with the wretched situation in last year’s Zimbabwean elections, with escalating government intimidation of the opposition, are unmissable. For most of the running time, Charman offers a compelling electoral quasi-thriller but eventually deviates oddly into an arid end-justifying-the-means debate. I objected to his hand-wringing liberal scruples when events take a surprising turn; or rather, I could have better tolerated them if the scenario he optimistically envisages had been the case in Zimbabwe.

Richard Eyre directs with his customary fluidity and Rob Howell’s elegant design uses hanging African matting to mark out the stage. It’s a marvellous central turn from the increasingly frenetic Chancellor, whose tensed shoulders and twitchy eyes suggest a woman who has toiled fruitlessly in high-pressure situations for too long. Charman’s warning is clear: the observers themselves must be observed.

Until 8 July (020 7452 3000, www.nationaltheatre.org.uk).