Dennis Knuckleyard is a hapless eighteen-year-old who works and lives in a second-hand bookstore. One day, his boss and landlady, Coffin Ada, sends him to retrieve some rare books, one of which, Dennis discovers, should not exist. A London Walk by Rev. Thomas Hampole is a fictitious book that appears in a real novel by another author. Yet A London Walk is physically there in his hands, nonetheless.

Coffin Ada tells Dennis the book comes from the other London, the Great When, a version of the city that is beyond time. In the Great When, epochs blend and realities and unrealities blur, while concepts such as Crime and Poetry are incarnated as wondrous and terrible beings. But, Coffin Ada tells Dennis, if he does not return the book to this other London, he will be killed.

So begins Dennis' adventure in Long London. Delving deep into the city's occult underbelly and tarrying with an eccentric cast of sorcerers, gangsters, and murderers, Dennis finds himself at the center of an explosive series of events that may endanger both Londons.