The River Line, Jermyn Street Theatre, London

Here is the kind of rediscovery at which Jermyn Street Theatre excels. Once renowned as a novelist and a playwright and for 15 years the chief theatre critic of The Times, Charles Morgan is no longer a name to conjure with. But Anthony Biggs's deeply absorbing revival of his forgotten 1952 play, The River Line, makes a strong case for its morally searching power and beauty and could help put this author back on the map.

Beginning and ending in a Gloucestershire country house in 1947, the piece wrestles with the ethical and spiritual implications of a devastating decision taken under extreme pressure in Occupied France during the war. Some reviewers of the original production scoffingly implied that the piece has the air of a bizarre John Buchan/T S Eliot collaboration in which a mini-thriller is sandwiched between bouts of over-rarefied philosophising on the subject of guilt and expiation. But Biggs and his fine cast demonstrate that the troubled soul-searching in the outer acts is, in its subtle, speculative way, just as intense and momentous as the fateful events in the central flashback to 1943. Here, hiding from the Nazis in a cramped granary near Toulouse, a group of Allied servicemen begin to suspect that one of their number is a German spy.

The figure whose spirit mountingly haunts the play is the charismatic young officer and poet, Heron (portrayed somewhat gauchely by Charlie Bewley of the Twilight films). Morgan clearly reveres this character for his gently inspiring serenity and the glowing way he embodies the injunction in the play's epigraph: "We must act like men who have the enemy at their gates, and at the same time like men who are working for eternity".

That double perspective on life informs the play's increasingly mystical conception of responsibility and atonement. A stoic acceptance that we each have to bear our flawed "responsibility in the predicament of the world" is the hard-won, pragmatic wisdom articulated by Lyne Renee's piercing Marie, the French woman who ran the Resistance group. But Philip, the former GI, who is played with a wonderfully compelling emotional transparency by Edmund Kingsley, comes to take the Greek tragedy view that even the sins we commit innocently or ignorantly demand expiation. Only the dead, though, have the right to forgive. Lydia Rose Bewley performs the climactic speech, where Heron communicates through her, with a quiet, shattering ardour that, like the play itself, puts you in profoundly persuasive contact with the operations of grace.

To 29 October (020 7287 2875)