On 21 October 1953, John Gielgud — the dream-voiced Shakespearean star of his generation, and knight of the realm — walked into a public lavatory off the Fulham Road. There, he exchanged a few nods and prods with a young man who winked at him — and triggered Britain’s highest profile prosecution for homosexuality since Oscar Wilde.

In Plague Over England, the entrapment of Gielgud by a “pretty policeman” is the centre of gravity for a play that swoops across the gay underworld of Fifties London — from the bushes of St James’s Park strewn with guardsmen to the Whitehall clubs where the crackdown on this “filth” is planned.

As Gielgud is convicted and shamed, a self-consciously gay London is emerging around him from the darkness of the lavatories and parks, trying to find light. As panicked men approach their doctors for help with their “disorder”, they are prescribed oestrogen to shrivel up their genitals, or electric shocks to burn their urges away. And in the middle of it stands Gielgud, an empty, air-headed genius, so great at donning masks because there is so little there. When he is caught, he mutters only a pitiful, desperate: “I’m sorry.” He sees no chance for political redemption: when somebody urges him to act like a suffragette, he laments: “Ah, but they had a just cause.” He only achieves eloquence once in the play when, lost and facing ruin, he turns to the audience and quotes Shakespeare’s Richard II: “What must the King do now? Must he submit?... Must he lose/ the name of king? O God’s name, let it go.” Gielgud is only himself when he is pretending.

The play’s author, Nicholas de Jongh, is best known as the acerbic theatre critic for the Evening Standard. But here, in his terrific first play, he sensitively traces a lost world: the dark waltz of cottaging where men circle each other for small signs of lust, and of politicians pledging to “eliminate homosexuality” from these crevices.

He manages to pull onto the stage the inner lives of a remarkable range of characters, and only fails in his depictions of the homophobes. They had complex, disturbing motives of their own — but here they are buffoonish caricatures motivated by only idiocy.

Of course, Michael Feast has an impossible job playing Gielgud. Nobody can bring back that voice — as pure and seductive as a Caribbean ocean — and nobody can bring back those eyes. But it is an extraordinary tribute that at times, there are flickers of the lost actor. As the play swings forward to the Seventies and the age of gay liberation, he captures the pathos of Gielgud still agonising. “The parade,” he says softly, “has passed me by.”

Fifty-six years after Gielgud ended up in court for a harmless act, it is a victory that “his crime” can now be re-enacted so compellingly on stage — in the knowledge the audience will damn the police, not the gay man.
Booking to 16 May. Box office: 0870 890 1103

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