Alan Bennett's new play, The Madness of George III (Lyttelton), is a history play, a political satire, a medical lampoon, and a portrait of a sad, difficult, lonely man. Polonius would have called it tragical-comical-historical-hysterical.

Of course, today we know that George III went "mad" for a few months in 1788 because an imbalance in his metabolism affected his nervous system; but that diagnosis would have been incomprehensible to the doctors of his time. Bennett's King says that he's not going out of his mind, but his mind is going out of him: a chillingly precise description of a state in which the sense of things slips away from your grasp. But that is only part of the story.

The King's madness was a political matter because the Prince of Wales supported Fox, Burke and the opposition; if he had been made Regent he could have dismissed the King's prime minister, Pitt the younger, and a lot of people would have been out of jobs. And so the King's doctors became the tools of politics, and the father's insanity fuelled the ambitions of the son.

I think Bennett makes the future George IV rather more of a nincompoop and less of a devious aggressor than he really was; and the ministers, turncoats, power-brokers and courtiers who crowd the play make it a little unfocused as a picture of hard political infighting. The role Fox (David Henry), Burke (Peter Laird) and Sheridan (lain Mitchell) played in all this was not very edifying, but they were more substantial figures than the shady operators Bennett has created. On the other hand I liked the shifty Lord Chancellor (James Villiers) and the austere; priggish, self-righteous Pitt (Julian Wadham), who stakes everything on sound money and needs only another five years to save the country. (Oh, yes?)

But essentially this is a one-character play, which is unusual in Bennett's work. Nigel Hawthorne's King is like a man who is his own jailer. He combines obtuse Teutonic arrogance with the invincible smugness of a thunderous English squire: a difficult interfering busybody who is infuriatingly well-informed and lets you know it, the stuff of which minor headmasters and pub bores are made. George's problem is that he happens to be King of England and is accustomed to being deferentially listened to. At the same time be is almost insufferably good-natured; and Hawthorne hints, with the greatest subtlety of English body-language, that behind the florid face and the small suspicious eyes there is a man who is not entirely sure of himself. It is the haunting portrait of a man who can find himself only when be loses his self-possession and finds refuge in being lost, and who is cured through self-knowledge. Hawthorne shines in the role like a cracked jewel.