Even a 400-room palace is not big enough to accommodate two ladies of the house -- especially a House of Lords like this one.

Two's company, three's a crowd for deliciously scandalous Georgiana Spencer, the Duchess of Devonshire and of this eponymous film, who was married off for her breeding ability at 16 to morose William Cavendish, Duke of Devonshire.

The Duke's digs in Derbyshire make Versailles look like the Motel 6. It takes the young Duchess (Keira Knightley) a while to adjust. But she does so soon enough, on her way to becoming revered as well as reviled for her wit, beauty and extravagant appetites -- drinking, gambling and politics, among them.

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'The Duchess'

3 stars = Good
Ratings explained

• Starring: Keira Knightley, Ralph Fiennes, Charlotte Rampling, Dominic Cooper.
• Rating: PG-13 for sexual content, brief nudity and thematic material.
• Web site: 'The Duchess'

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Georgiana (1757-1806), in her heyday, was the queen of English society and fashion (while the real Queen Charlotte was busy with her mad-hatter hubby George III, bearing his 15 children and the burdens of his insanity). Georgiana also was a devoted mother (of too many girls, not enough boys) and activist ahead of her time -- all the while being betrayed by her dull, dour Duke (Ralph Fiennes) and her best gal-pal Elizabeth Foster (Hayley Atwell). Bess sleeps not only with Georgiana's husband (and Georgiana herself), but then literally moves in with them for a permanent menage a trois.

Truth may not be stranger than fiction, but it's just as strange: Like her great-great-great-great-niece Diana (nee Spencer) in the 20th century, Georgiana was a glamorous darling of both her own aristocratic set and the hoi polloi, trapped in a loveless relationship. The film's marketing has milked those Princess Di links: "There were three people in her marriage ..."

No need. Knightley's alluring charm and vivacity have carried every character she has played -- from "Pride & Prejudice" (2005) through two "Pirates of the Caribbean" versions (2006, 2007) and "Atonement" (2007) -- and serve her well here. Her duchess portrayal is also well served by the ice-cold Fiennes, whose watery blue eyes and nonverbal hauteur are incomparable. Charlotte Rampling provides an additional assist as canny, calculating Lady Spencer, the Duchess' dowager mum.

But then there's Dominic Cooper in the crucial romantic-historical role of Charles Grey, whose real-life association with her advanced his career. I'm sorry, but this Earl Grey is not my cup of tea. With his bad headgear and 5 o'clock shadow, he looks too Mediterranean to be a credible British lover, let alone prime minister-to-be. By the time they both let their hair down, literally and figuratively, in a bona fide sex scene, it's too late for much organic chemistry.

Gyula Pados' sumptuous cinematography ratchets up the production values, but director Saul Dibb is constrained by a soap-operatic screenplay, based on Amanda Foreman's best-selling biography. The bottom line is a kind of a forced-feminist bodice-ripper with romantic intrigue galore, enjoyable but not significant.

That's unfortunate, because the actual Duchess was truly a player in her country's affairs, outwitting many patriarchal potentates around her. The film gives short shrift to her work as an anti-Tory reformer -- a supporter of the American and French revolutions, of ending the slave trade, and of other sweeping changes of the rising Whig Party.

The real wig party in "Duchess" involves the architectural wonder of Knightley's hairdos, whose heights rival the Empire State Building. In one close encounter of the candle kind, her towering coiffure becomes a towering inferno, which just goes to show -- if nothing else -- that the vanity of virtuosic verticality ought not veil the value of a good flame-retardant.