The trailer makes this costume drama look like a dude’s nightmare, all hotness buried in yards of lace and crinoline. But in telling the true (enough) 18th-century story of young Georgiana, the handful who got stuck marrying the haughty, unfaithful Duke of Devonshire (Ralph Fiennes), the filmmakers plugged in a live wire. That would be Keira Knightley, an actress with a gift for bringing humor and heat to period pieces (Atonement, Pride and Prejudice). Based on Amanda Foreman’s biography of the Duchess, the film, directed perfunctorily by Saul Dibb, underlines every parallel it can find between Georgiana and Princess Diana, starting with the Duke’s affair with Bess Foster (Hayley Atwell). Knightley meets every challenge of the role as the Duchess becomes a devoted mother, an infamous fashion plate and a favorite of the common people. She too takes a lover, in the person of Charles Grey (Dominic Cooper), with whom she has a bastard child. She even finagles a truce with the Duke. Fiennes gives a quietly devastating performance in a part that could have been played for pure villainy, especially in a scene of marital rape. Instead, this consummate actor finds the details that help us understand the forces, noble and ignoble, that forged the Duke’s character. Make no mistake, the film belongs to Knightley. There’s a fire in her eyes that won’t be extinguished, and it keeps us in her corner despite, or more likely because of, the mischief she sparks. It’s Knightley who makes The Duchess a royal treat.