Based on Amanda Foreman’s biography of Georgiana Spencer, the Duchess of Devonshire, Saul Dibb’s costume drama tells how Princess Diana’s 18th-century ancestor (played here by Keira Knightley)—a naive ingénue married off in her teens to a fornicating icy stiff (get the parallel?)—grew into a politically sophisticated woman. Cruising lightly over Georgiana’s activism, Dibb firmly turns the spotlight on her love life, in which she must come to terms with a ménage à trois at home and a passionate love affair with Charles, Earl of Grey (an incongruously laddish Dominic Cooper), a future prime minister and namesake of the posh tea. But for all its frisky high jinks, brocaded homes, and creamy bosoms, The Duchess is a tragedy about the terrifying vulnerability of even the richest women in a society that deprives them of property rights. As a tale of mature self-sacrifice, the movie would be almost unbearably moving were it not for Knightley’s insubstantial performance, which allows her to be fatally upstaged by Ralph Fiennes—who, despite having played many a stiff quite stiffly, has bags of fun here playing Georgiana’s husband: a jerk, but also a man of his time who’s not oblivious to the happiness of the women in his life.