Roland Michell (Aaron Eckhart, Nurse Betty, Your Friends and Neighbors) is a US scholar researching the life of 19th century English poet Randolph Henry Ash (Jeremy Northam, The Misadventures of Margaret). He finds original letters suggesting that Ash may have had an affair with another poet at the time, Christabel LaMotte (Jennifer Ehle). When he meets with Maud Bailey (Gwyneth Paltrow, The Royal Tenenbaums, Shakespeare in Love, Running with Scissors), a London expert on LaMotte, she is skeptical. LaMotte was believed to be a lesbian and lived with companion Blanche Glover (Lena Headey, Imagine Me and You, Mrs. Dalloway, "Band of Gold") for years.

When Roland reads Blanche's journals, he finds that Christabel did receive letters and poetry from Ash. The film moves between present day and the 1800s as we see what is found in letters and diaries playing out.

Christabel attempts to cut off contact at one point, writing about Blanche, "It is a chosen way of life in which I have been wondrously happy and not alone in being so. I have chosen a way, dear friend, I must hold to it." But it does not end, causing only heartache for Blanche.

Roland and Maud follow the trail of evidence and begin to fall in love themselves as we see Christabel and Ash do the same. Unfortunately, regardless of the interesting plot and lovely historical settings, Possession is another film in which a woman leaves her lover for a man. To top it off, Blanche commits suicide in stereotypical style.