Ravishing as a Gainsborough portrait, rapt by its subject's real estate, high hair and higher style, "The Duchess" chronicles a momentous decade in the life of 18th-century fashion plate and political hostess Georgiana, Duchess of Devonshire.

Like her celebrated descendant, Princess Diana (her great-great-great-great niece), the glittering duchess was beloved by everyone except a glum spouse who preferred the company of loyal dogs and a submissive mistress to that of his independent-minded celebrity wife. Any resemblance between the dour duke and a certain present-day prince is strictly intentional.

In the opening scenes of filmmaker Saul Dibb's highly varnished tableau, Keira Knightley glows as the coltish 17-year-old, affianced to England's most eligible bachelor (Ralph Fiennes), permitting herself to believe her mother's suggestion that she is his true love. To the pragmatic duke, who turns to ice in the face of his fiancee's ardent fire, she is a brood mare in corsets.

Unloved in her marbled and marquetry prison, Georgiana sought affection outside it. Garnishing chapeaus with ostrich plumes, lacing her wit with wine, she was the life of every party, including that of the Whigs. The duke was the pooper, trailing boredom in his shroudlike cloak.

As Dibbs -- and co-screenwriters Jeffrey Hatcher and Anders Thomas Jensen -- tell it in the film adapted from Amanda Foreman's biography, the moody duke and merry duchess were an epic mismatch, as unthinkable a pair as Mr. Rochester and Elizabeth Bennett -- or Voldemort and Hermione Granger.

The duchess was a soaring creature, so the earthbound duke clipped her wings -- and in the film's most chilling scene, also her elaborately festooned bodice. (While the dresses, designed by Michael O'Connor, are smashing, the marital rape is shocking.)

When the duchess befriends another of her species, Bess Foster (Hayley Atwell) -- estranged from her own husband and barred from seeing her children -- Georgiana opens her heart and home. No sooner does Bess gently instruct her new friend that sex is something to be enjoyed, not feared, than the interloper becomes the duke's mistress.

In the spirit of what's sauce for gander is gravy for goose, Georgiana, cardsharp and gambler, proposes a deal. The duke can carry on with Bess if the duchess can take her childhood friend, Charles, Earl Grey (Dominic Cooper as the future prime minister and man for whom the tea was named), as her lover.

The bemused expression on the duke's face -- why would I deal when I hold all the cards? -- cracks the duchess' jaw. And the rumble of Gothic thunder permanently overtakes the charming chamber divertimento.

At this pivotal moment, the film's sympathies seem to subtly shift from duchess to duke -- from how resourceful and exuberant the duchess is, to how demanding and petulant. I wondered if female screenwriters would have differently dramatized this sequence. See: Sofia Coppola's "Marie Antoinette." (Coincidentally, the French queen and duchess were intimates.)

While attempting to depict the injustice of patriarchy, the filmmakers would appear to guiltily enjoy its spectacle. And when the duchess is dealt what might be called the "Sophie's Choice" hand, they furthermore imply that Georgiana never really rebounded from losing this game of marital poker. Just because the filmmakers and their financiers deny Georgiana a third act doesn't mean she did not have one. While I much liked "The Duchess," this portrait feels unfinished.

"The Duchess"

HHHHH

At the Grandin Theatre. Rated PG-13 for sexual themes, brief nudity and discreet sexual violence. One hour, 50 minutes.