It was one of the most compelling historical biographies of recent years, and a film adaptation has always seemed inevitable: Amanda Foreman’s Georgiana, Duchess of Devonshire related the story of a tragic heroine apparently custom-built for the big screen. Georgiana Spencer became Duchess of Devonshire in 1774, aged 17 (her husband was 26). She was expected to produce a son and heir for the duke, but instead she bore two daughters. The marriage deteriorated after the duke seduced Georgiana’s friend Lady Elizabeth (Bess) Foster, and they lived in a ménage à trois for a decade. But by then the intensely sociable Georgiana had established herself as a taste-maker in society, and a political mover and shaker for the Whig Party. She bore a child by the future prime minister, Charles Grey, and ran up huge gambling debts.

(Georgiana did, in 1790, give birth to a son, though this is not covered in the film. Bess also bore the duke a son and a daughter and married him after Georgiana died. He then found himself a new mistress.) In her time Georgiana became a genuine celebrity whom the common folk seemed to love. She was, if you will, the people’s duchess.

The film’s producer, Gabrielle Tana, who bought the rights to Foreman’s book 10 years ago, said, “I never thought the film wouldn’t happen. But it’s been a long journey.” Keira Knightley plays this 18th-century It girl (Tana and Foreman, who are friends, were adamant that a British actress had to play Georgiana).

Ralph Fiennes is the distant duke. In the same spirit, the film’s sizeable budget (£13.5 million) came only from Europe: Pathé Films and BBC Films bankrolled the picture between them.

It was shot last summer on a nine-week whistle-stop tour of some of Britain’s most impressive stately homes — including Chatsworth, the Devonshire family’s ancestral seat. “When you are offered a role like this,” Knightley said, “you don’t turn it down.” The Duchess is set to come out on September 5.

The Duchess sees Keira Knightley return once again to period drama after a string of starring roles in The Edge of Love, Atonement and Pride and Prejudice	She plays Georgiana, Duchess of Devonshire, in the life story of the controversial 18th-century aristocrat, based on Amanda Foreman's best-selling novel
Ralph Fiennes is the Duke, but their marriage is an unhappy one and scandal is not far away	A poisonous ménage à trois develops between the couple and Lady Elizabeth Foster (Hayley Atwell)
Whig politician Charles Grey (Dominic Cooper) becomes infatuated with Georgiana and she goes on to mother his child	Bess (Elizabeth foster) becomes good friends with the Duchess — but also her husband’s lover
Due to her beauty and liberal politics, the Duchess becomes a favourite of the people, in spite of her fraught private life	An ancestor of the late Diana, Princess of Wales, parallels might be drawn between the two women's dramatic lives in the public eye