Tilly and Oskar embark on an impromptu trip to the Library of Congress in Washington, DC, in search of an elusive group of bookwanderers called the Archivists, who they believe are the key to restoring the balance and saving the British Underlibrary. Along the way, the friends realize that something strange is happening in the world of bookwandering: books are disappearing, people are forgetting their favorite stories, and stories are trying to drag Tilly into them. With a mysterious set of clues and some surprising new faces, Tilly and Oskar will need the help of some of their dearest fictional friends to be able to solve the mystery . . . and save bookwandering forever.