The novel opens in the autumn of 1986 in London. Roland Michell is a researcher who has recently completed his PhD studying the Victorian poet Randolph Henry Ash, but now finds himself frustrated by the lack of a permanent academic job. While looking through a book owned by Ash, he finds two drafts of letters addressed to an unknown woman whom Ash seems to have been struck by and sought to continue a conversation with. Roland is intrigued and steals the letters from the library without telling anyone he has found them.
Working from what he knows of Ash, Roland develops a hypothesis that the woman addressed in the letters is Christabel LaMotte, a less well-known Victorian writer who has been recently rediscovered by feminist scholars. He travels to Lincoln to meet with Maud Bailey, a scholar who specializes in researching LaMotte. Maud allows him to look at some documents which offer small hints that a correspondence between Christabel and Ash may have taken place in 1858 and 1859. A relationship between the two would be a significant discovery, as Ash has always been perceived as a happily married man, and Christabel (who never married, and lived with a female companion for an extended period) has been widely considered a lesbian. Roland confides in Maud about the letters he has discovered, and she becomes equally consumed by a desire to know what happened. Maintaining secrecy around their research is vital, since a group of scholars including James Blackadder, Leonora Stern, and especially Mortimer Cropper aggressively pursue any knowledge or discoveries concerning either poet.
By chance, Roland and Maud discover a collection of letters hidden in the house where Christabel lived in the later part of her life. These letters document the correspondence between the two poets as they gradually fell in love with one another over a period of months. The letters end abruptly in the spring of 1859. Roland, with his knowledge of Ash’s biography, knows that in June 1859 the poet spent a month travelling in Yorkshire, and wonders if it possible that Christabel went with him. Roland and Maud go together to retrace this journey, and become increasingly convinced that hints contained in the writings of both authors indicate that they were together during this time. Scenes set in the time the poets were alive confirm that Christabel and Ash did secretly travel together as lovers.
Roland and Maud now face a new question: if Christabel was with Ash, what happened next? By coincidence, Maud’s friend and collaborator Leonora Stern has been informed of a discovery in France which might provide information about Christabel. Maud and Roland follow up on this information and discover documents describing how Christabel fled to France after discovering she was pregnant, and secretly gave birth. The whereabouts of the child were never determined. By now, the other scholars have also pieced together information about the relationship and are determined to unlock the secret of what became of the child. Cropper is the most aggressive, and the others begin to suspect that he plans to dig up the grave of Randolph Henry Ash, since he believes additional documents have been buried along with the poet.
On October 15, 1987, Roland and Maud, along with a group of allies who have been drawn into the research project, confront Cropper and his helper as they try to rob the grave. Nonetheless, they all agree to open and read the letter Cropper retrieved. The letter is from Christabel to Ash: when she learned he was dying, she sent him a letter finally explaining that she had given birth to a daughter who has been raised by her sister. Their daughter, May Bailey (an ancestor to Maud) has lived a happy life without ever knowing who her true parents were. Ash’s wife was supposed to give him the letter, but she never did, and it was buried unopened. Meanwhile, legal experts have confirmed that as a descendant of both poets, Maud will inherit all the letters and documents, and can make them available for new scholarship. With their project finally completed, Maud and Roland admit they have fallen in love with each other while working together on their research. The novel ends with a hopeful implication of their beginning a relationship together.