Warning - spoilers ahead in this review!
The Duchess is a period movie staring Keira Knightly as Georgiana, the Duchess of Devonshire. Georgiana is a Spencer who marries the most eligible bachelor in England, William, the Duke of Devonshire. As the new Duchess of Devonshire, Georgiana becomes a trend setter and the most popular woman in England. For all of her wealth and popularity, Georgiana finds that her husband is "the one man in England not in love with his wife." She befriends Lady Charlotte, only to have her husband cheat on her with Charlotte in her own home. Georgiana tries to find true love with Charles Grey, but is unable to break free from her marriage and strange living situation with William and Charlotte.
Overall, I thought the movie was mediocre. It did have one heart wrenching moment when Geogiana is forced to give up her love child. It also had some good points on how inequality among sexes as William is allowed to have a live in mistress, but Georgiana is not allowed to have her passion to the side. Overall it was inconsistent (Georgiana can't leave her husband because she doesn't want to abandon her children yet she can abandon her love child. What??) and mearly brushed the surface of Georgiana's life without going into details. It would have been a better movie if it would have really shown the interesting things Georgiana did in her political life and really played up the love between her and Charles. I found myself not really caring about the characters.
I think if you are really interested in the subject, skip the movie and instead read the book, Georgiana: Duchess of Devonshire by Amanda Foreman. It is a fantastic book and really gives you the depth about Georgiana that is missing from the movie.