It’s doubtful that there’s a more sheerly entertaining playwright in the English language than Bennett. He was the quiet bloke in the ’60s quartet that revolutionized comedy with “Beyond the Fringe” (Jonathan Miller, Dudley Moore and Peter Cooke were the noisy ones). In the colonies, Bennett is known mainly for plays seen on public TV like “An Englishman Abroad,” about the spy Guy Burgess, and a superb series called “Talking Heads,” in which Maggie Smith played a vicar’s alcoholic wife. Bennett is an old-fashioned (meaning timeless) humanist who can deal with subjects like traitors or desperate wives without a trace of sentimentality or pretentiousness. If Tom Stoppard or David Hare had written a play about George III, it would have been honeycombed with theses about madness, power and politics. Fair enough. But Bennett says simply that he thought “George III might be fun to write about.”

“The state of monarchy and the state of lunacy share a frontier,” says Dr. Willis, one of four physicians trying to cure the king. This group is itself a beyond-the-fringe troupe of hapless medicos: one concentrates on the royal bowel movements, another keeps poor George retching with emetics, another blisters him with hot glasses, and one pops the proud potentate into straitjackets and a restraining chair. George oscillates between seeking comfort from his Queen Charlotte and making salacious passes at a lady-in-waiting. He tries to strangle his wastrel heir, the Prince of Wales, who schemes with opposition politicos to take power, while the prime minister, Wilham Pitt, scrambles to keep the government functioning.

It’s a tragicomic spectacle with echoes of Aristophanes, Moliere and, finally, Shakespeare. In a scene both poignant and richly comic, the king orders his retinue to join him in reading from “King Lear.” Shakespeare turns out to be the king’s most effective therapist, as if playing Lear purges George of the poisons of human despair and imperial hubris. Bennett shows kingship as an ordeal, seductive but absurd and dangerous to one’s health. Nigel Hawthorne’s amazing performance as George has a Dickensian flair and blare. He’s a ruler who loses power over himself, a Humpty Dumpty chasing the pieces of his shattered psyche. Hawthorne, who won a 1991 Tony award for “Shadowlands” might well have won another if this play were coming to New York. It’s Broadway’s best no-show of the season.