The Duchess" follows, with reasonably favorable results, the formula for English period dramas: great costumes, great sets, great passions, ridiculous wigs.

And the sex is pretty good, too.

The story of Georgiana, the Fifth Duchess of Devonshire, has been covered in three books in the past 20 years alone (no doubt in part due to her ancestral connection to Diana, Princess of Wales), and those who read them must have thought her story would make a fine movie.

With "The Duchess," they were right.

Keira Knightley stars as Georgiana Spencer, the 18th-century arbiter of fashion taste in London, the first woman to campaign for a political candidate and, if the film is accurate, the model for Lady Teazle in Richard Sheridan's classic comedy "The School for Scandal."

Shortly after her marriage to the Duke of Devonshire, the most important man in England after King George, Georgiana learns two important things: The duke's primary interest in life is producing a male heir, and he has no other interest. The dull but mercurial duke is played by Ralph Fiennes, who plays boring very well -- for all the character's money and influence, he almost disappears into the tapestry.

The duke is not, strictly speaking, faithful. And it is not long before the coltish, flirty Georgiana's eye also begins to wander. Meanwhile, Georgiana's mother (Charlotte Rampling) urges a course of fortitude and resignation.

It's standard costume-drama stuff, with two distinguishing twists: One is that it is true, and the other is that it is told with a hint of modern feminism. This political filter may be anachronistic, sometimes distractingly so, but it gives the film a point of view often lacking in others of the genre.

Audiences looking for such things will also find trenchant echoes of Georgiana's life with that of her famous descendant, Diana. Both were young when they married high-ranking men, both outshined their unexciting husbands as celebrities of the day, both set a standard for fashion and, most important, both tolerated their husbands' infidelity with a long-standing lover.

Anglophile Americans may be content simply to soak in the sumptuousness of the production, from the intricate costumes of Michael O'Connor to the grandeur of the sets in manor houses to the golden photography of Gyula Pados (though he relies perhaps too much on wide-angle shots) to the inevitable music by Rachel Portman.

Even with the visual splendor, the film begins to drag toward the end; you may find yourself checking your watch and calculating how much time is left. But in the end, given the potency of the true story and the intriguing personalities explored, it is time well-spent.