Possession opens up in the London Library in the autumn of 1986. There, a postdoctoral researcher named Roland Mitchell is poring through a dusty old book that was once owned by the celebrated nineteenth-century poet Randolph Henry Ash. Suddenly, Roland comes across two drafts of a letter to an unnamed woman who was definitely not Ash's wife. Who could she have been? Seized with burning curiosity and an unexpected flare-up of scholarly "possession," Roland steals the letters, and decides to discover their addressee.
Roland's sleuthing soon turns up a likely candidate—another nineteenth-century poet, named Christabel LaMotte. On the advice of a charmingly wolfish colleague, Roland travels to Lincoln to meet Maud Bailey, one of the world's leading LaMotte scholars. Maud soon finds herself just as captivated as Roland, and she asks if he'd like to visit the place where Christabel LaMotte once lived.
In a rural Lincolnshire village called Croysant le Wold, some conveniently heroic behaviour on Roland's part scores them an invitation to Seal Court, the manor house where Christabel LaMotte spent the last 20+ years of her life. There, Roland and Maud discover the academic scoop of the decade—two whopping bundles of love letters between Christabel LaMotte and Randolph Henry Ash.
As Roland and Maud wait for another invitation to Seal Court (they haven't been allowed to read all of the letters yet), they try to follow whatever other leads they can find. Finally, they're given the green light to return to Seal Court in January. When they do, they sit together in Seal Court's frosty library for days, slowly reading through the earth-shattering (okay, discipline-shattering) correspondence.
After their January visit to Seal Court, Roland and Maud go their separate ways until the spring. In April, after the winter term has finished, they get together again to catch each other up on the ideas they've had, and theories they've formulated, since their last encounter. Roland believes that Christabel LaMotte may have travelled to North Yorkshire with Randolph Henry Ash in the summer of 1859, but they have no way of knowing for sure.
As Maud and Roland's quest for information starts to come together, things are getting tricky in their personal lives. Roland's girlfriend, Val, is convinced that Maud and Roland are having an affair, and Maud's ex-boyfriend, Fergus Wolff, is trying to follow Maud and Roland's every move. When Maud visits London to track down a lead, Fergus confronts her in the basement of the British Museum, demanding to know what she and Roland are up to.
Despite being watched by prying and jealous eyes, Roland and Maud decide to take a research trip to North Yorkshire. They figure that if they trace the route that Randolph Henry Ash was known to have taken, they might find some evidence that Christabel LaMotte had been there with him, too. The trip doesn't disappoint: they return feeling sure that the nineteenth-century lovers had gone to North Yorkshire together.
Things start to move quickly now, as Possession gets ready to tie a whole bunch of threads together. Rival scholars and colleagues alike are catching the scent of Roland and Maud's discovery, and the feathers are really flying. As Roland and Maud start to see their secret quest become a very public affair, they decide to do something completely unexpected—they disappear to Brittany together, to follow up on some new information that's just recently come to light.
In Brittany, Roland and Maud read through a series of journal entries by one of Christabel LaMotte's distant cousins. The journal reveals that Christabel fled to Brittany after her trip to North Yorkshire with Randolph Henry Ash, and, in the spring of 1860, she gave birth to a child. Problem is, no one knows what happened to the baby.
Roland and Maud spend three weeks in Brittany, hanging around by the seashore. When they realize that their colleagues James Blackadder and Leonora Stern have tracked them down, they hightail it back to England. There, Roland camps out on Maud's couch for weeks, until an unexpected telephone call from a lawyer named Euan MacIntyre brings them back into the thick of things.
Euan MacIntyre—who just so happens to be dating Roland's former girlfriend, Val—believes that Maud may be the rightful owner of the Ash-LaMotte correspondence, and he's prepared to help her prove it. As Maud and Roland talk things over that night, Maud gets another unexpected phone call—this one from their mutual colleague Beatrice Nest. Beatrice tells Maud that their academic rival, Mortimer Cropper, is planning to dig up Randolph Henry Ash's grave in search of buried documents.
Roland, Maud, Euan, and Val head to London the next day, and there they gather together with Beatrice Nest, James Blackadder, and Leonora Stern. Together, the group comes up with a plan to thwart Mortimer Cropper and prevent him from making off with whatever is buried in Randolph Henry Ash's grave.
Things come to a head in a rural English cemetery, where Possession's heroes succeed in claiming the loot that Cropper wanted to steal. The "loot" is an old black specimen box containing private documents and mementoes, and inside it is an unopened letter from Christabel LaMotte.
As the novel's characters sit together in a rural inn, Maud reads Christabel's final letter to Randolph aloud. As she does, she makes a huge discovery: Christabel LaMotte was her great-great-great-grandmother!
After that, things quickly come to a close for the novel's twentieth-century characters. With their quest complete, Roland and Maud head to bed together for the very first time—after they finally confess their lovey-dovey feelings for each other, natch.
The novel itself wraps up with a Postscript that ties up one last thread. In it, we learn that Randolph Henry Ash did get the chance to meet his daughter before he died. As Possession's narrator tells us, he came across her by accident one afternoon while walking through a meadow near her home, and, after recognizing her, traded a crown of flowers for a lock of her golden hair.