For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Possession's first act begins with Roland Mitchell's discovery of Randolph Henry Ash's letter to an unnamed woman, and it takes us all the way up to the fateful afternoon when Roland and Maud Bailey discover the whole kit and caboodle of the Ash-LaMotte correspondence at Seal Court. Of course, because they haven't yet been allowed to read all of the correspondence, their "quest" to learn the secrets of the nineteenth-century love affair has only just begun.
Act II
Our second act introduces us to the novel's principal villain, the entertainingly awful Mortimer Cropper, who starts to put some pressure on Roland and Maud. As they investigate the love affair between Christabel LaMotte and Randolph Henry Ash, their own lives get messier and messier. Things really come to a head when their colleagues catch a whiff of their discoveries: pretty soon, Roland and Maud's private "quest" has become a very public affair.
Act III
The novel's third and final act takes place in dimly lit, cozy inn near Hodershall, not too far from the cemetery where Randolph Henry Ash is buried. Together with the whole company of people who've been hot on the trail of the Ash-LaMotte love affair, Roland and Maud examine the contents of the black specimen box that the tricksy Mortimer Cropper dug up from R. H. Ash's grave. As the novel draws to a close, its twentieth-century characters finally learn how Randolph and Christabel's story came to an end.