I... just don't even know where to begin with this one. Initially reads as a fairly straightforward historical mystery, when a series of gruesome and bestial murders hits 1880s Edinburgh. Self-absorbed police Inspector Groves, who often seems more interested in how a case will be presented to future readers of his memoir-in-progress, suspects that the crimes are tied together through an odd young woman, Evelyn Todd, but resistance to his questioning by Evelyn herself, as well as the associates of the murdered men, stymies his investigation.
Meanwhile, Professor McKnight and Mr. Canavan, whose unexpected friendship has developed around a shared love of books and philosophy, take up their own investigation into Miss Todd's background and experiences, hoping to discover what links her to the deaths of such a varied cast of characters, the mighty and the seedy.
Evelyn's vast imagination, it ensues, is capable of somehow manifesting her desires as she sleeps. Better still, it seems that it unleashes a silent "lodger" who occupies her mind and is freed only when she is unconscious. What and who is that "lodger"? how did it come to share her mind, and why are these particular people its targets? The last quarter of the book becomes increasingly phantasmical, requiring the reader to set aside any requirements for a tidy wrap-up.