In some ways Shaw was a skilled theatrical tart: he could always turn tricks to satisfy the customers. But behind the crowd-pleasing lay a missionary fervour, and even in this early 1894 comedy, deftly revived by Sam Walters, you can detect Shaw's loathing of war and passionate belief in the nonconformist conscience.
On one level, the play offers a simple confrontation between romance and realism. During the 1885 Serbian invasion of Bulgaria the operatically inclined Raina Petkoff gives shelter to a fugitive Swiss mercenary, Captain Bluntschli. He is the kind of practical working soldier who admits to fear and regards chocolate as more useful than cartridges in the heat of battle. Against him Shaw sets Raina's fiance, Major Sergius Saranoff, a dashing exhibitionist who leads heroic cavalry charges against machine guns. When peace is declared and Bluntschli returns to the Petkoff household, it doesn't take much effort to see that the quiet professional will win out over the posturing paladin.
Since we no longer believe in romantic military heroism, you could say the play has dated. What keeps it alive, paradoxically, is Shaw's own passion. Amid all the farcical carryings-on about a borrowed greatcoat, Bluntschli suddenly reveals that a Serb colleague was pointlessly burned to death in a woodyard fire; in that moment alone you hear Shaw's hatred of the futile cruelty that underlies the sham heroics of war.
Even more revealingly, the Byronic Sergius defines the truly brave man as one "who will defy to the death any power on earth or in heaven that sets itself up against his own will and conscience". You could hardly have a clearer statement of Shaw's belief that courage is a moral rather than a physical quality. It is worth sitting through the earlier whimsy for such moments of untrammelled feeling.
Howard Saddler is outstanding as Bluntschli, lending him just the right quality of shrewd omniscient sanity, while Susan Salmon and Tim Weekes extract what fun they can from the idea of a pair of mismatched servants. Two other actors also make impressive professional debuts: Leah Muller, swooning to the remembered sounds of Verdi's Ernani, is a suitably rhapsodic Raina and Ben Warwick is all fire and fervour as Sergius. There may be greater Shaw plays, but few that show so well how the entertainer masked the moralist.
• Till October 14. Box office: 020 8940 3633.
