Although The Brass Butterfly is a comedy, its central concerns are similar to those which appear in the novels. The first performance in Great Britain of the play was at the New Theatre, Oxford, on 24 February 1958. It was directed by Alastair Sim, who also played the Emperor. The play, in three acts, is set in Capri, ‘sometime in the 3rd century AD’. Mamillius, the affected and foppish illegitimate grandson of the Emperor, is vainly trying to amuse himself, when two strangers arrive. They are Phanocles, an inventor, and his daughter Euphrosyne, who have come to show the Emperor Phanocles’ inventions. The Emperor is particularly impressed with Phanocles’ pressure cooker, but the inventor himself is more interested in demonstrating his explosive launcher and his ability to convert a galley into a steam ship. Although the Emperor is deeply suspicious of change, and says, ‘Phanocles, in my experience changes have seldom been for the better,’ he is persuaded to give Phanocles an old barge which he converts into the warship, Amphitrite. The heir designate, Postumus, alarmed at what he takes to be treachery, rushes to Capri to depose Mamillius and the Emperor. During the ensuing violence, Amphitrite sinks, set ablaze by the slaves who feared being made redundant by steam. Postumus is killed by an explosion, because the quick-thinking Euphrosyne, with whom Mamillius has somewhat inevitably fallen in love, has removed a safety device, the ‘brass butterfly’ of the title. In the closing moments of the play, Phanocles demonstrates his own favourite invention: the printing press. Initially impressed by the potential of the invention, the Emperor swiftly revises his opinion and becomes alarmed at the prospect of the proliferation of paper. In order to suppress the invention for as long as possible, he makes Phanocles an ambassador and sends him, with his printing press and gunpowder, on a slow boat to China.

It might seem that Phanocles is being mocked for his rationalist vision of the world, but the Emperor is portrayed as a sensualist whose age now permits him to indulge only in the pleasure of eating, hence his delight in the pressure cooker.