The film depicts the ordeal of King George III whose bout of madness in 1788 touched off the Regency Crisis of 1788, triggering a power struggle between factions of Parliament under the Tory Prime Minister William Pitt the Younger and the reform-minded Leader of the Opposition Charles James Fox.
At first, the King's habits appear mildly eccentric, and are purposely ignored for reasons of state. The King is seen as being highly concerned with the wellbeing and productivity of Great Britain, and continually exhibits an encyclopedic knowledge of the families of even the most obscure royal appointments. In fact, the King is growing more unsettled, largely over the loss of America. The King's eldest son George, Prince of Wales, aggravates the situation, knowing that he would be named regent in the event the King was found incapacitated. George chafes under his father's repeated criticism, but also hopes for regency to allow him greater freedom to marry his Catholic mistress Mrs. Fitzherbert. George also knows that he has the moral support of Fox, who is eager to put across an agenda unlikely to pass under the current administration, including abolition of the slave trade and friendlier relations with America. Knowing that the King's behaviour is exacerbated in public, the Prince arranges for a concert playing the music of Handel. The King reacts as expected, interrupting the musicians, acting inappropriately towards Lady Pembroke, Lady of the Bedchamber, and finally assaulting his son.
The King's madness is treated using the relatively primitive medical practices of the time, which include blistering and purges, led on particularly by the Prince of Wales' personal physician, Dr. Warren. Eventually, Lady Pembroke recommends Dr. Francis Willis, an ex-minister who attempts to cure the insane through new procedures, and who begins his restoration of the King's mental state by enforcing a strict regimen of strapping the King into a waistcoat and restraining him whenever he shows signs of his insanity or otherwise resists recovery.
Meanwhile, the Whig opposition led by Fox confronts Pitt's increasingly unpopular Tory government with a bill that would give the Prince powers of regency. Meanwhile, Baron Thurlow, the Lord Chancellor, discovers that the Prince was secretly and illegally married to his Catholic mistress. Thurlow pays the minister to keep his mouth shut, and himself tears out a record of the marriage from church rolls.
The King soon shows signs of recovery, becoming less eccentric and arrives in Parliament in time to thwart passage of the Regency bill. Restored, the King asserts control over his family, forcing the Prince to "put away" his mistress. With the crisis averted, those who had been closest to the King are summarily dismissed from service, including Dr. Willis. During conversations with Pitt, the King appears more at ease and in control of himself. He is less antagonized by America, but also shows signs that his insanity remains. A final message states that the King likely suffered from porphyria, noting that it is an incurable chronic condition and is hereditary.
Nigel Hawthorne as King George III
Helen Mirren as Queen Charlotte
Ian Holm as Francis Willis
Amanda Donohoe as Lady Pembroke, Lady of the Bedchamber
Rupert Graves as Colonel Greville
Geoffrey Palmer as Doctor Warren
Rupert Everett as George, Prince of Wales
Jim Carter as Charles James Fox, Leader of the Opposition
Julian Rhind-Tutt as Frederick, the Duke of York
Julian Wadham as William Pitt the Younger, Prime Minister
Anthony Calf as Lord Charles FitzRoy
Adrian Scarborough as Fortnum
John Wood as Lord Chancellor Lord Thurlow
Jeremy Child as Black Rod
Struan Rodger as Henry Dundas
Barry Stanton as Sheridan
Janine Duvitski as Margaret Nicholson
Caroline Harker as Mrs. Fitzherbert
Roger Hammond as Baker
Cyril Shaps as Pepys
Selina Cadell as Mrs. Cordwell
Alan Bennett as a backbench MP whose speech is interrupted by everyone running out to see the King
Nicholas Selby as Speaker