In late 1788 the king babbled incessantly, jumped on the daughter of the duke of Marlborough and allegedly greeted a tree in Windsor Park as the king of Prussia. His subsequent mania was partly provoked by barbarous treatment. The royal doctors bled him and peered at the contents of the royal chamber pot. Unknown to them, he was suffering from porphyria, a physical disease with the out- ward symptoms of madness. Enter Dr Willis, who regarded mentally disturbed patients as horses to be broken in. When the king complained, he was put in a strait- jacket, strapped in a chair and gagged. At no point was he medically examined. Nigel Hawthorne secured an Oscar nomination for his roller-coaster ride to the verge of mental disintegration and back. Other strong performances are Helen Mirren's passionately loyal Queen, Julian Wadham's glacial Pitt, Ian Holm's menacing Willis and Rupert Graves' guileless Greville.

Here is a vanished era of royal politics. Ministers are appointed by the king, not the electorate. When he loses his senses government stops and political infighting starts. Pitt's attempt to hang on to power without royal backing becomes a pivot of the drama, as his opponents move in for the kill. The royal court is recreated in all its grandeur and discomfort. Etiquette is agony. Royalty sits while everyone else stands. Courtiers cannot look at the monarch when they speak to him and have to walk backwards they take their leave. When he starts to dash around erratically, they have to run backwards. Court life is focused on a personally active monarch: as his madness advances it disintegrates. A valid historical contrast is drawn between the majesty of George as a sovereign and the scant respect for him as a man once the trappings are torn away.

The period detail is compulsively watchable and mostly accurate. Costumes are a riot of powdered wigs and scarlet uniforms, with the camera dwelling lovingly on rich fabrics and orders of chivalry. Dazzling colour depicts monarchy in glory. With the onset of royal madness, it drains away and reverts to wintry blacks and greys ' only to return in a blaze of exultation on the king's recovery, with Handel in Hallelujah mode on the soundtrack.