BORN to one of the richest families in England, she married into a clan that was even wealthier and more eminent. Her husband was nearly a decade older and never pretended to feel anything close to the romantic love she yearned for. Instead he took a mistress. But if he didn’t want his wife, just about everyone else did. She was a huge celebrity with a unique sense of style that was widely copied. Above all, she was devoted to her children.

Does the story line sound familiar?

Diana, Princess of Wales, was not the first member of the aristocratic Spencer family to win the heart of her country but not her husband. In 1774 her ancestor Lady Georgiana Spencer married the Duke of Devonshire, who had been considered the most eligible bachelor in England. Their sad union is the focus of “The Duchess,” which opens Sept. 19.

Based on Amanda Foreman’s 1998 best-selling biography, “Georgiana: Duchess of Devonshire,” the film concentrates on only about 10 years in Georgiana’s life, starting with her arranged marriage at 17. It’s a compelling story, but period films like “The Duchess” are always a hard sell to mass audiences, even when they have big names like Keira Knightley in the title role and Ralph Fiennes as her husband. With so many uncanny parallels in the two women’s lives, pushing the Diana connection (as the British Web site, theduchessmovie.co.uk, and trailer do) is an obvious marketing tactic.