Stop me if you've heard this one: A young beauty of noble descent marries a boring peer who wants the marriage but also wants his mistress. What ensues is an unhappily-ever-after story -- in this case, about the celebrated Georgiana Spencer, 18th-century ancestor of the late Diana, Princess of Wales. And her apparent prototype in celebrity, popularity and misery.

Directed by Saul Dibb, "The Duchess" shows that if Dibb knows nothing else, he knows what he's got in star Keira Knightley: She seems to get as much close-up time as Bette Davis got in her entire career. We're not complaining.

Knightley is magnetic. She can also act. Hollywood probably doesn't know what to do with her.

She's a beautiful, heroic and engaging Georgiana, the soon-to-be wife of the fairly abominable Duke of Devonshire (Ralph Fiennes). The bewigged D wants nothing more than a son to carry on his name.

Her naivete is painful, even more so in retrospect, after the Duke proves himself a brute. Under their marital roof, he seduces Georgiana's friend Bess (Hayley Atwell) with the promise of helping her regain custody of her children. Then he refuses to let Bess leave. Georgiana is forced into a shotgun menage a trois.

Fiennes has the unhappy task of making a dullard interesting. He succeeds. That his wife gives birth to two girls, and mothers his illegitimate daughter, fails to impress him. Georgiana is his superior in every regard but money and power, and that is certainly clear to the public, whose adulation elevates her into an icon of 18th-century celebritude.

Simon McBurney's Charles Fox (Whig politician then British secretary of state) is a gnarly little Georgiana fan who kicks the film into gear whenever he's on screen. McBurney is such a singular presence he could be cast as Satan himself.

It's too bad there's not more substance to "The Duchess," because there's lots of acting and, as is required of a Brit-styled period piece, lushness galore.

"The Duke of Devonshire," someone says mid-film, "is the only man in England not in love with his wife." Everyone loves Georgiana, or will, except maybe women who are pregnant, or have been: By the end of the film she has given birth to four children, and she still looks like Keira Knightley.

-- John Anderson (Sept. 26, 2008)

Contains sexual content, fleeting nudity.