The year is 1949, the city London. Amidst the smog of the capital is Dennis Knuckleyard, a hapless eighteen-year-old employed by a second-hand bookshop. One day, on an errand to acquire books for sale, Dennis discovers a novel that simply does not exist. It is a fictitious book, a figment from another novel. Yet it is physically there in his hands. How ? Dennis has stumbled on a book from the Great When, a magical version of London beyond time and space, where reality blurs with fiction and concepts such as Crime and Poetry are incarnated as wondrous, terrible beings.
But this other London must remain a secret : if Dennis cannot find a way to return this book to where it belongs, he risks bizarre and disastrous repercussions, such as his body being turned inside out (or worse). So begins a journey delving deep into the city's occult underbelly and tarrying with an eccentric cast of sorcerers, gangsters and murderers - some from legend, some all too real, and each with plans of their own.
Soon Dennis finds himself at the centre of an explosive series of events that may alter and endanger both Londons forever.