After the exciting rediscovery of Emlyn Williams’s Accolade at the Finborough last year, here comes an even more revelatory excavation from the same period: Charles Morgan’s The River Line (1952) is a brilliantly constructed, almost mystical, story of love, guilt and reconciliation at a Gloucestershire dinner party in 1947.

The first and third acts are separated by a flashback to a country granary near Toulouse in 1943, where a group of soldiers are following the “river line” – the clandestine escape route across occupied France – helped by the Resistance.

Not only is Morgan’s writing expertly high-flown – with great slabs of poetry included – his plotting is superb; he’d “tried out” the story in a 1947 novel, but hit on the idea of this gripping second act, with its murderous climax, as an animated anecdotal interlude between the Gloucestershire scenes.

Morgan (1894-1958) was The Times theatre critic for fifteen years before turning to novels and plays. On the evidence of The River Line, it’s alarming just how forgotten he’s become.

Anthony Biggs’ production, with a stark but functional setting by Rhiannon Newman Brown, does him proud on this tiny stage, with a real live brother and sister – Charlie Bewley from the Twilight Saga films, and Lydia Rose Bewley – as a suspected undercover agent and his emigrating sister Valerie.

In truth, both Bewleys are slightly gauche and awkwardly articulating on stage, but they are buoyed along by energetic performances from Edmund Kingsley as Valerie’s American wannabe “intended” and Lyne Reneeas Marie, the French Resistance operative, who is now married to Christopher Fulford’s impressively haunted army officer.

Fulford, a really fine actor, is the crux of the show, and there’s good support, too, from the redoubtable Dave Hill as Marie’s shuffling peasant father, Alex Felton as a vivaciously callow young private and Eileen Page as Valerie’s godmother, rejoicing in the perfect English landscape on a summer night in July.

Harold Hobson ranked The River Line alongside TS Eliot’s The Cocktail Party, and it certainly shares that play’s poetic immanence; not only does Morgan tell a damned good story, he worms his way, skilfully and remorselessly, under the skin of a generation condemned to live in the present while imprisoned in the past.