Film: Keira Knightly in "The Duchess"

As eye candy, "The Duchess" offers up everything you'd want in an 18th century tale: towering wigs, painted faces, expansive gardens, grand estates and, of course British aristocrats in formal clothes behaving badly. The good news is that it's considerably more interesting than the conventional romantic yarn—passionate Duchess follows her heart and rebels against stifling social conventions!—that the trailer makes it out to be. The twists and turns of the legendary Duchess of Devonshire's life (the subject of Amanda Foreman's well-regarded biography "Georgiana: Duchess of Devonshire") were far more baroque and twisted than your standard bodice-ripping yarn, and up to a point director Saul Dibb's undeniably enjoyable movie is willing to subvert some of the clichés of the standard Hollywood period drama. Yet for a number of reasons "The Duchess" isn't all it could have been. It's fun, but falls short of fabulous.

Georgiana (Keira Knightley), the kind of bright and headstrong girl that always gets called "spirited," is married off to the very rich and even more powerful Duke of Devonshire (Ralph Fiennes) at the age of 17. Her primary duty is to give him a male heir. She's quite excited about the merger, but the Duke is one extremely cold fish, more passionate about his dogs than his beautiful bride and so deeply enmeshed in noblesse oblige that he expects his wife to tolerate all his peccadilloes, which include romps with the chambermaid, inviting his illegitimate daughter into the family and, the biggest affront to Georgiana's pride, taking her best friend Bess (Hayley Atwell), a well-born divorcee, as his mistress and installing her as a permanent member of the household.

Trapped in a ghastly marriage with a husband whose indifference turns to anger when she produces one girl child after another, the Duchess finds other outlets for her energy. The real Duchess became a figure of great fame, popularity, and scandal (she was a distant relation of Lady Di, a fact the studio is pushing hard). Her fashions set the styles of the day; her parties were the toast of society; and she became a figure of great political influence, campaigning for the Whig party, which meant endorsing the abolitionist Charles Gray (played in the film by Dominic Cooper) the childhood friend who became her lover.

This affair, which enrages her husband and will force her to make a painful choice in life, becomes the focus of the later part of the movie. It's easy to understand why Dibbs, who worked on the screenplay with Jeffrey Hatcher and Anders Thomas Jensen, would keep the focus on the erotic and romantic entanglements of the Duke, the Duchess, his mistress and her lover, but the other aspects of Georgiana's remarkable life get short shrift. The movie gives us little sense of how she rose to such prominence and popularity, and even less sense of her political passions and influence. Reading the production notes on the film you learn that she was, in addition, a novelist, poet, musician and amateur scientist—none of which emerges on screen—and that she was addicted to gambling and had a voracious appetite for drink. This doesn't come through at all.

All of this suggests a much more robust and fascinating character than the elegant young woman Knightley gives us. The fact that her Duchess, even after giving birth to four children, never adds a pound to her svelte, 21st century body, is indicative of her lack of psychic heft in this role. Knightley is not a bad actress, but she only scratches the surface of a part that requires a depth of experience that's not yet in her range. She's at times in danger of being overshadowed by the complex, ambiguous Bess, who is both her betrayer and ally.

The triumph of "The Duchess" is the Duke. Fiennes plays the odious nobleman with such subtlety, wit and perfect control that he steals the movie outright. A viciously pragmatic man with no illusions about himself, he gives the part a surprising humanity without ever softening him. It's a portrait of what the perks of power can do to a man's soul, and he cuts a figure that's comic and terrifying and understandable all at once.

The negative chemistry between Fiennes and Knightley is far more electric than the sexual chemistry between her and Cooper, whose role is cut from standard romantic fiction cloth. They go through the motions of passion without ever quite igniting it in the audience. "The Duchess" is at its best at the long, formal breakfast table, where the Duke, his wife and his mistress must share an eternity of awkward meals as they play their assigned roles in the Duke's kinky 18th century design for living. At such juicy moments we could be watching a Pinter play about power, gamesmanship and sexual humiliation, played out in powdered wigs in front of eavesdropping servants and high-society gossips. Ultimately, "The Duchess" doesn't do justice to the magnificent contradictions of its formidable title character. But it's elegantly mounted and never dull—and always easy on the eyes.