A journalist kills himself in his Norfolk cottage, leaving a note which even he does not fully understand: "I can't bear it any more, though I don't know what 'it' is." The writer is James Mossman (Ben Chaplin) and it is he who stands before us, expressing his confusion. Spanning the years 1963-71, Nicholas Wright's The Reporter uses Mossman's turbulent life and death as a lens through which to view the cultural and political upheavals of the time. The deaths of Churchill, Dimbleby and Britain's grandeur; Vietnam; swinging London'; the vanishing of deference.

As we see from Mossman's personally disastrous interview with Harold Wilson (Patrick Brennan), the veteran field journalist was a proto-Paxman, refusing to take No comment' bluster for an answer. But his "regrettable habit of speaking his mind" was anathema in the BBC's golden age of journalistic detachment. "We mustn't indulge in the open candour we demand of others,"tuts a bitchy Robin Day (Paul Ritter).

Amid all the personal chaos, the play manages a surprising amount of humour (albeit often through gritted teeth): spying "bears the same relationship to international politics as cottaging does to sex". Humour is just one of Mossman's facets, though. He is touching in his unconventional friendship with batty novelist Rosamond Lehmann (Angela Thorne); foolish in his love for Louis (Chris New); business-like but generous towards Daniel, his producer (Leo Bill); ruggedly professional with his cameraman, Marko (Aleksander Mikic; pictured above, with Chaplin).

Meanwhile his boss, Ray Ray (Bruce Alexander), desperately mediates between Mossman and others who don't find him so agreeable. It is in its treatment of these personalities and the BBC's internal politics that the play packs its contemporary punch, reflecting a modern preoccupation with what the author calls "the culture of lies". As recent publications on institutional leftism in the BBC attest, media bias and complicity is once again under close scrutiny - the play is full of familiar questions about journalistic sanctimony, unaccountability and partiality.

Though politics is a key element in "the 'it' portfolio", The Reporter is essentially an intimate drama, and the Cottesloe a neat space for it. The outside world literally whirls around the disoriented Mossman in the fast and fluid scene-changes, historical context flashing up on to the screens and walls of Richard Eyre's production. Clever lighting and design soften the cuts from reel footage to stage action, most effectively in the Panorama scenes where even the actors seem to be operating in black-and-white.

Alas, though, it was the ever-present shades of grey that did for Mossman, and The Reporter is sad and troubled overall. But, for all that, with frequently varied personnel and a pacey production the play is easily digestible, and Chaplin's tenacious and complex performance is a pleasure to watch.

The Reporter continues until June 2. Box office, telephone: 020 7452 3000 (www.nationaltheatre.org.uk).