Kiera Knightley plays 18th century aristocrat Georgiana, Duchess of Devonshire in this muddled but eye-filling saga of an ancestor of Diana, Princess of Wales, who shared her status as a fashion icon, heartbroken wife of a man in love with someone else, and object of intense public fascination.

Georgiana was still a teenager when she was told that one of the most sought-after catches in England wanted to marry her. Silly girl, she thought it meant he loved her. But the Duke thought of marriage as something between a political alliance and animal husbandry. All he cared about was her ability to produce a male heir. The property law of the time depended on continuing the male line (as readers of Pride and Prejudice well know). And, as an affecting scene near the end shows, the Duke (Ralph Fiennes) is as constricted by the conventions of the era as she is.

This creates the opportunity for a soap opera in period clothes, with mutual adulteries and children born to other partners. Georgiana (known as “G”) also struggles with the betrayal of her closest friend (“Brideshead Revisted’s” Hayley Atwell) and the agonizing choice between her great love, a politician (“Mamma Mia’s” Dominic Cooper), or her children with the Duke.

It never quite comes together because G’s life was too big and varied to fit on screen. She was a fashion icon and an influential figure in politics. She was a devoted mother and an “all for love and the world well lost” libertine. She was strong and weak, intelligent and foolish, loyal and disloyal, practical and a dreamer. Though Knightley is game, the film falters in pomp and loses her under the feathers and jewels.