A rash of mysterious happenings on the London to Brighton road has terrorized the county of Sussex, and the police are at a complete loss as to how to account for the disappearances and injuries which are taking place. Burgess Clymping and his sister Ann, who live near the scene of the incidents, soon find themselves embroiled in the affair, which begins as a straightforward mystery, but quickly takes on a more sinister meaning. With the help of their friend, Lincoln Osgood, the trio manages to piece together the puzzle, to reveal a horrifying truth: one which imperils the lives and souls of all those it touches.

Originally published in 1919, The Door of the Unreal is one of the first true werewolf classics, and helped to pave the way for the 'golden age' of the genre, which flourished in the two decades following the publication of Gerald Biss's novel. At the time he wrote the novel—his one foray into supernatural fiction—Biss was largely covering uncharted territory: most of the now-familiar conventions of the werewolf tale had yet to be set in stone, and the author was not working to the type of narrow pattern which constrained many writers of vampire tales in the years following the publication of Dracula. As Stefan Dziemianowicz writes in his introduction: 'The Door of the Unreal . . . reminds us of a time when one of horror's classic themes had not yet been forced into a rigid mould shaped by genre expectations, and when the plot of a horror tale was limited by nothing more than the imagination of writers like Gerald Biss.'