The troubled, headline-making marriage of Prince Charles and the late Princess Diana had nothing on the tumultuous relationship of Diana’s 18th-century ancestor, Georgiana Spencer, and her husband, William Cavendish, the fifth Duke of Devonshire. In The Duchess, we can see some of the genetic origins of Diana’s style and spirit, but the late, beloved 20th-century royal’s life was idyllic compared to Georgiana’s wedlock headlock. I Married an Ass might be a more bluntly accurate title for this feminist portrait of a pre-pre-feminist misalliance.

Keira Knightley, today’s reigning queen of the costume drama, is radiant in the title role, despite all the angst. We first meet her as a mischievous 16-year-old in 1774, flirting with a group of young men including the handsome Lord Charles Grey (Dominic Cooper), just before her mother reveals the match she’s made for her with the imposing Duke (Ralph Fiennes). Georgiana is delighted with the arrangement, but her husband proves to be anything but a warm, gentle lover on their wedding night. The Duke grows increasingly colder and distant as Georgiana fails to conceive a male heir for him, instead (horrors!) giving birth to two daughters. Meantime, the Duke sees it as no imposition on his bride when he brings his own illegitimate young daughter into the household.

Georgiana finds solace in a close friendship with the plainspoken Lady Elizabeth Foster (Hayley Atwell of Brideshead Revisited), a bond that’s broken when she discovers Bess en flagrante with the Duke. Her husband not only continues the affair, but moves Bess and her children into Devonshire House. Ultimately, the Duke does get his male heir—after raping Georgiana in a burst of fury at his ungrateful spouse.

Amidst her travails, the charming and intelligent Georgiana becomes a social sensation, both a fashion icon and a major player in the growth of the progressive Whig Party. One of those rising Whigs happens to be Charles Grey, the love of Georgiana’s life. When the Duchess becomes pregnant with Charles’ child, the temptation to change her fate is overwhelming. But the Duke has a powerful agenda of his own.

Director Saul Dibb, whose only previous theatrical feature was the contemporary youth crime drama Bullet Boy (2004), has crafted an eye-catching production, abetted by the talented Hungarian cinematographer Gyula Pados and production designer Michael Carlin (The Last King of Scotland). Dibb and co-writers Jeffrey Hatcher and Anders Thomas Jensen, adapting from the best-selling biography Georgiana by Amanda Foreman, tend to soft-pedal the Duchess’ documented drinking and gambling; Georgiana seems like the most demure of libertines, the better to maintain sympathy for our heroine. The 23-year-old Knightley proves again that she has the beauty, poise and acting chops to carry a film, while Fiennes manages the more difficult assignment of finding the haunted humanity within the essentially despicable Duke. Atwell is terrific as the conflicted, practical Bess, and Cooper (fresh off Mamma Mia!) makes an appealing suitor when not trapped under an unbecoming wig. Always watchable, Charlotte Rampling lends her timeless grace to the role of Georgiana’s formidable mother.

Husbands and boyfriends, be especially diplomatic when accompanying your dates to The Duchess. This lavish look back at a royal pain and his beleaguered bride is a chick flick with anger issues.