1949, the city London. Dennis Knuckleyard is a hapless eighteen-year-old who works in and lives above a second-hand bookshop. One day, on an errand to retrieve rare books, Dennis discovers a book that simply does not exist. It is a fictitious book, yet it is physically there in his hands nonetheless. How? Dennis has stumbled on a book from the Great When, a dark and magical version of London that is beyond time. There, epochs blend and reality blurs with unreality, while concepts such as Crime & Poetry are incarnated as wondrous and terrible beings. If Dennis does not take this book back to the other London, he will be no more. So begins a journey delving deep into the city’s occult underbelly and tarrying with an eccentric cast of sorcerers, gangsters, and murderers, some legendary, some all too real, and all with their own intentions. Soon Dennis finds himself at the centre of an explosive series of events that may alter and endanger both Londons forever.