Ralph Fiennes and Keira Knightley star in the costume drama The Duchess, based on the life of the scandalous, 18th-century English aristocrat Georgiana Cavendish, Duchess of Devonshire. (Paramount Vantage)

The ad campaign for The Duchess shows Keira Knightley in 18^th-century sky-high wigs and hoop skirts the size of hotdog cart umbrellas. But Knightley, as Georgiana Cavendish, the Duchess of Devonshire, is not the star of the ads. Diana, Princess of Wales, is the product being pushed — she appears in black-and-white still photos that call on our dusty reserves of sympathy with the tagline: “There were three people in their marriage.”

It’s tacky to exploit the image of a doomed woman who had a love-hate relationship with her celebrity to sell a movie about a doomed woman who had a love-hate relationship with her celebrity. But the parallels are striking: both were fashionistas trapped in arranged marriages haunted by the spectres of their husbands’ mistresses. If that’s not enough for you, both Knightley (playing Georgiana) and Diana (playing Diana) possess beauty, ego and toothpicks for bones.

But the retro hard sell isn’t necessary. Neither Paris Hilton-in-Frocks nor Diana: The Prequel, The Duchess is a costume drama quietly attentive to the grand absurdities of royal life. Adapted from a biography of Georgiana by Amanda Foreman, the film is enough of a literary inquiry that it almost makes you forget Knightley’s recent decline from actress to cosmetics model.

Georgiana is a teen bride to the laconic, greying duke (Ralph Fiennes). While uncoiling her endless ribbons on their first night together, he murmurs about the silliness of women’s dress. His new wife answers, “You have so many ways to express yourselves, but we have only hats.” The duke looks at his wife with genuine shock – The power of speech! In a woman! — and her fate is sealed. Even as Georgiana becomes a wildly popular figure in public life, the duke remains steadfastly indifferent to her considerable charms. To him, she is less an It Girl and more a broken soda machine that fails to produce male heirs, instead popping out girls again and again.

Georgiana ages out of a giddy youth, finding a calling in motherhood and gambling (now that’s a maternal figure I can dig, Ms. Palin), and bitterly accepting the great divide in her marriage. Her mother, played with wonderful comic resignation by Charlotte Rampling, is the Greek chorus telling her to lower her expectations; for even the most famous women, power is between the legs.

The couples’ various infidelities result in a marital arrangement that would be considered forward-thinking today: the duchess’s best frenemy, Lady Foster (Hayley Atwell), is also the duke’s lover, and they all shack up in a sprawling castle à la Fleetwood Mac.

From left, Lady Spencer (Charlotte Rampling), the Duke of Devonshire (Ralph Fiennes), "Elizabeth "Bess" Foster and Georgina, Duchess of Devonshire (Keira Knightley) engage in high society life in domestic drama in The Duchess. (Nick Wall/Paramount Vantage)

There can be something laborious about period pieces that dig for modern allegory, but no one had to look very far for a relevant sexual politics lesson in The Duchess. Director Saul Dibb never forces the feminist theme, but allows the story a rare breeziness. Those hoping for the battle scenes and stampeding extras that usually accompany heritage England pieces will be let down by the small scale of the film. It’s really just a sad domestic drama with fabulous costumes.

Knightley can be a brittle actress, too content to jut her jaw and simply grace us with her beauty. She’s much fiercer in The Duchess, and it’s a hell of a part. But Fiennes makes the film, giving contour to a character that might have been a simple villain. He has evil moments, especially in the bedroom, but he, too, seems burdened by his station.

What scares this near-mute about his wife is her flexibility, her openness to the world. She takes to liberal politics, using her following to promote the Whig Party (Sienna Miller wouldn’t do that) as she awakes to something called “freedom.” Excitement about America and the French Revolution buzzes in the background of the film. The duke just wasn’t made for these times. He’s compulsively drawn to order, and grows more confused than angry that order is falling apart, as it will without an heir. Yet Fiennes locates comedy in this bad guy, and just a touch of silliness, as when he calmly delivers this line, after the drunken missus’s updo catches fire at a ball: “Please put out Her Grace’s hair.”

The duchess eventually falls in love with Charles Grey (Dominic Cooper), future prime minister of England. But their affair is the flattest note in the film; this awkward, slight creature just doesn’t seem capable of matching the duchess’s desires.

Already, some historians are grumbling that the film is a hollow, inaccurate portrait, one that plays down the real Georgiana’s political allegiances in favour of her romantic ones. So be it. There’s substance enough in any film that redeems motherhood – a category much appropriated and denigrated of late – by showing a strong woman whose most potent lust is for a life outside the prescribed parameters of highest-society wife.

The Duchess opens Sept. 19.

Katrina Onstad is the film columnist for CBCnews.ca.