James Mossman, a brilliant foreign reporter and presenter on the BBC’s Panorama current affairs programme during its heyday, committed suicide in 1971, leaving a note in his Norfolk cottage: “I can’t bear it any longer, though I don’t know what ‘it’ is.”

Nicholas Wright’s new play, directed by former NT boss Richard Eyre, attempts to define what the “it” was in the form of a wide-ranging investigation presented by the dead reporter himself. Although Mossman, charismatically played by Ben Chaplin, emerges as an interesting, conflicted character who exchanged the dangers of the Vietnam War for the security of a television studio, there are no final answers to the question. That knotty ambiguity in the play is both its unusual strength and its slight weakness as theatre.

In a programme note, Wright explains how he worked as an assistant floor manager at the BBC and knew Mossman slightly, having also once known Mossman’s lover, the Canadian potter Louis, slightly better.

Working on fragmentary information, much as he did with his play about van Gogh, Vincent in Brixton, Wright creates an enthralling tapestry of guilt, grief and radical and BBC politics in an age of crumbling public propriety. The result combines elements of Tom Stoppard’s Rock ‘n’ Roll and Peter Morgan’s Frost/Nixon, with a smattering of Alan Bennett’s speciality in homosexual spies, those heavy-drinking gay deceivers.

Mossman – a tall, rangy man with a piercing gaze and a frank, disarming manner – was once likened by a colleague to a cross between Lawrence of Arabia and Cassius. Chaplin looks nothing like him yet catches him exactly, especially in his dryness, watchfulness and emotional integrity.

Rob Howell’s design recreates the grey television studio where Robin Day (a brilliant impersonation by Paul Ritter), known as “cruel glasses,” has replaced the Churchillian gravitas of a sick Richard Dimbleby and where Mossman allows his anger about Britain’s support for the Americans in Vietnam to boil over in an interview with Harold Wilson (Patrick Brennan, who conveys the Yorkshire bluffness of the former Labour PM but not his guile or slipperiness).

The resulting internal fracas (Bruce Alexander is the hilarious embodiment of shifty, dry-mouthed apparatchik life in the BBC corridors) sees Mossman demoted to arts programmes. But his involvement with Louis (Chris New, fresh from his exciting debut in Bent) has already led him to follow a soft feature story in San Francisco instead of dashing to Los Angeles to cover the Robert Kennedy assassination.

After Louis is found dead from an overdose of barbiturates, Mossman seeks reconciliation with him through his friendship with the novelist Rosamond Lehmann (a wonderfully distracted Angela Thorne), whose psychic novel has made a great impression on him.

Lehmann talks about “crossing over” instead of dying, and when Mossman tells Robin Day that he is thinking of doing so, Day assumes he is going to ITV. There are many such good jokes in an evening that may not pack a final punch but which clearly offers the best new play of the year so far. And Gillian Raine is a small, cameo delight as Mossman’s real mother; his other legendary “mother” is the formidable BBC floor manager Joan Marsden (Tilly Tremayne).

- Michael Coveney