The Duchess, of course, has the tagline, "there were three people in her marriage," which was playing on something Princess Diana, the descendant of Georgiana, the duchess in question, had said about her own marriage. The three people in question being Georgiana, her cold, detached husband the Duke (played very well by Ralph Fiennes) and her best friend, Bess, who comes to live with them as the Duke's live-in mistress. Fair enough. In the Amanda Foreman biography on which the film is based, this little threesome is the focus. However, in the film, they choose to highlight the fourth person in the marriage--the future Earl Grey, the love of Georgiana's life. Dominic "History Boys" Cooper makes an attractive Grey though he doesn't seem experienced enough to play this role very well. So far, then, four people--and that's before you include the duke's beloved pet dogs.
The basic plot is this: Georgiana very excitedly marries the older Duke (she seems more excited about the idea of becoming a duchess than with the Duke himself), spends seven years failing to produce an heir (although has two daughters), while the Duke has multiple affairs, latterly with Georgiana's best friend Elizabeth Foster. She (reasonably, perhaps?) thinks that if he can do it, so can she and starts carrying on with Grey. "No," sez the Duke, "this is the 18th century, dear, and you are forgetting that there are plenty of double-standards to comply with. I will continue to shag Bess but you are not allowed to be with this Grey chap. Now, come here and let's produce an heir." The film portrays the Duke as abusive and taking Georgiana by force, although, of course, these standards don't really make sense in an 18th century context. In the end, he makes her choose between Grey and her children--she picks Grey and then starts to read the letters from the enfants that the Duke has thoughtfully left behind, and changes her mind and goes to confine herself to a lifetime of misery and lost opportunities.
While this is all recorded in the biography, there are other elements that seem more important, such as Georgiana's addiction to gambling and repeated, crippling debts and the relationship between Bess and Georgiana. The film has one, vaguely sapphic scene but Foreman highlights throughout the extremely deep friendship between the two and although she cautions that it doesn't make sense to talk about a heterosexual vs homosexual relationship between the two, people did question the closeness of their friendship; furthermore, in reality, Georgiana's friends and family questioned her devotion to Bess, who they felt was using her for various ends and that Georgiana was too gullible to see it.
None of the background of friendship comes off very well in the film, though, where Georgiana sees her husband flirting with Bess, goes to introduce herself, becomes best friends with her, then goes post-modern when she finds out Bess and the Duke are sleeping together. The same is true in The Edge of Love, where Keira this time plays Dylan Thomas's other woman to Sienna Miller's wife: the two meet in a pub (Keira being surprised that Dylan has a wife), Sienna sez, "I'm not sure I'm going to like you," Keira sarcastically sez, "well, I'll be on the edge of my seat" and in the next scene, the two are shacked up together in Wales, sharing baths (it was the war--they had to conserve water) and secrets. Not that Sienna was any less angry when she found out that dear Dylan's interactions with Keira were present tense rather than simply past, although again, the friendship between the two women was strong enough that they could remain best chums, despite the weird, entangled situation in which they found themselves.
Again, in The Edge of Love, there were four people in Keira's marriage--Dylan, Sienna and her husband, William. I couldn't really bring myself to care too much, though, as the patchy (at best) Welsh accents were very distracting and none of the characters were particularly sympathetic--Sienna's "oh gosh, I am wounded. oh noes!" acting made it hard to feel too much pity for her sweet, if doormatty, character, and the flaws of the others were present in abundance. The film was trying to be far artier than it ought (I did like the soundtrack, though) and by the end, I definitely felt that one Keira film per weekend was definitely more than enough.