It tells the true story of George III of Great Britain's deteriorating mental health, and his equally declining relationship with his eldest son, the Prince of Wales, particularly focusing on the period around the Regency Crisis of 1788–89.

Two text panels at the end of the film note that the colour of the King's urine suggests that he was suffering from porphyria, adding that the disease is "periodic, unpredictable and hereditary."

King George III's bout of madness in 1788 touched off the Regency Crisis of 1788 and triggered 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.

He is deeply concerned with the wellbeing and productivity of Great Britain and exhibits an encyclopaedic knowledge of the families of even the most obscure royal appointments.

Led by the Prince of Wales' personal physician, Dr. Warren, the King is treated using the medical practices of the time, which focus on the state of his urine and bowel movements and include painful cupping and purgatives.

At his farm in Lincolnshire, patients work to gain “a better opinion of themselves.” He observes to an equerry “To be curbed, thwarted, stood up to, exercises the character.” When the King insults him, foully, he is strapped into a chair and gagged.

When the Prince has the King transferred to Kew, Charlotte watches as her beloved, bearded and wearing a soiled diaper and a straitjacket, struggles against being put in the coach.

The Whig opposition confronts Pitt's increasingly unpopular Tory government with a proposal that would give the Prince powers of regency.

Alan Bennett insisted that director Nicholas Hytner and actor Nigel Hawthorne should be cast in the film version, after having acted in the play.

The site's consensus states: "Thanks largely to stellar all-around performances from a talented cast, The Madness of King George is a funny, entertaining, and immensely likable adaptation of the eponymous stage production.

"[12] Reviewing for Variety, Emanuel Levy praised the film, writing: "Under Hytner's guidance, the cast, composed of some of the best actors in British cinema, rises to the occasion... Boasting a rich period look, almost every shot is filled with handsome, emotionally charged composition".

[13] John Simon of The National Review wrote, "The Madness of King George III has survived the transfer from stage to screen, and emerges equally enjoyable on film."