I finally had the time to watch this movie. It's a poignant narrative based on the life of the the 18th century Duchess of Devonshire, Georgina.
The movie opens with a scene in 1774 in England where we're introduced to the free-spirited, young and vivacious Georgina (Keira Knightley) with a hint of some mutual adoration between her and Charles Grey (some unknown actor Dominic Cooper). Unbeknownst to her, a match has been arranged by her mother and the Duke of Devonshire, William (Ralph Fiennes).
The innocent Georgina marvelled in awe upon arriving at the sprawling compounds of the castle but what seemed like a handsome match turned out to be her greatest torment as William is portray as an unfaithful, unfeeling husband whose sole purpose of consummation is for procreation. Not only did she put up with his promiscuous ways (with maids and even her best friend), she had to bring up one of his illegitimate child as her own and was constantly reminded of her failure to produce a male heir (she had two daughters, two stillborn sons).
The re-entrance of Charles Grey into her life opens her eyes to all that she had missed. She said in one scene, "I fear I have done some things too early and others too late." I believe she meant that she got married too early and discovered love too late.
She did eventually produce a male heir for William but also gave birth to an illegitimate child, Eliza, with Charles, who was taken away from her and brought up as Charles' niece (he never knew, I think).
Torn between love and responsibility, passion and duty, Georgina was imprisoned in her loveless marriage til her death.