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Sex without love was also the norm for Georgiana Cavendish, 18th century socialite and chosen wife of the Duke of Devonshire, whose semi-interesting story is told in Saul Dibb’s semi-interesting The Duchess. At its opening shot of Keira Knightley’s back, ringlets bouncing and ornate train dragging along a perfectly manicured lawn, you can’t help but think you’ve seen this period piece before. Nearly two hours later, you’ll feel like you’ve been sitting for three, and with that first instinct pretty much confirmed.

The most compelling aspect of The Duchess isn’t that the royal couple becomes a triple, nor that Georgiana (Knightley) was pressured by society and even her own mother (Charlotte Rampling) to—and this may come as a shock—put on a happy face and perform her wifely duties despite her loveless marriage. More intriguing is Georgiana’s fashion-plate reputation and general popularity despite her husband’s preference for the company of other women or even his dogs; as one character puts it, “The Duke must be the only man in England not in love with his wife.” Georgiana was the people’s princess and pop icon two centuries before Lady Di came along—from the Duchess’ own bloodline, Diana Spencer being a distant niece.

Screenwriters Dibb, Jeffrey Hatcher, and Anders Thomas Jensen (working from Amanda Foreman’s biography of the duchess) also inject parallels to the current election into the story, as Georgiana uses her celebrity to promote the Whig party and its leaders bellowing about change. The film, against all reason, actually perks up when politics are discussed, proving that its characters are thinking beings and not just models for flouncewear. (But wigs and hats, unfortunately, are given more prominence here than Whigs, with Knightley’s getting increasingly absurd, from merely hyper-inflated poofs to bizarre Afro-like wedges and, always, towering headgear.)

The source of Georgiana’s misery is Ralph Fiennes, whose Duke William is socially awkward and monosyllabic, sealing his marriage negotiations with the teenager’s mother with a wan, “So be it, then.” The Duke’s sole motivation for marrying was to produce a male heir, and despite Georgiana’s beauty, their sex life is mechanical, their romance nonexistent. When G, as William calls her, fails to immediately produce a son, things go from civil to resentful. Among other humiliations, William saddles Georgiana with a daughter from a previous liaison so she can “practice her mothering skills,” and he later starts an affair with her best friend, Bess (Hayley Atwell), who had moved in with them—and never left.

Though The Duchess’ pace is slow, its acting is as good as can be expected, and better in Knightley’s case: Finally, she does more here than look aristocratic, with moments of Natalie Portman–esque impishness as well as nuanced expressions of heartbreak. Fiennes’ casting is atypical and he’s thorough as a bore. Yet, well, his character’s a bore. After a while, William becomes truly hateful, and the trouble is that Georgiana, no matter how depressing her straits may be, becomes hard to sympathize with, too, seeming to get dumber as the years go on instead of learning how to play the game. Blame the hats and rent Atonement.