A dark and beguiling tour through the streets of a magical London by the Sunday Times bestselling master of modern fantasy, Alan Moore The year is 1949, the city London. Dennis Knuckleyard is a hapless eighteen-year-old who works and lives in a second-hand bookstore. One day, on an errand to retrieve rare books, Dennis discovers that one of them does not exist. It is a fictitious book, yet it is physically there in his hands nonetheless. How? It comes from the Great When, a dark and magical version of the city that is beyond time. There, epochs blend and realities and unrealities blur. If Dennis does not take this book back to the other London, he will be killed.

So begins a journey delving deep into the city's occult underbelly and tarrying with an eccentric cast of sorcerers, gangsters, and murderers, many of whom have their own nefarious intentions. Soon Dennis finds himself at the center of an explosive series of events that may alter and endanger both Londons. Mystical, magnificently written and hilarious, The Great When is Moore's most imaginative work yet and the first in the fantastic new Long London series.