'I was in the wrong London. It was horrible. I'd fallen in by accident, and it was all alive and trying to eat me. '

The year 1949, the city London.

Hapless second-hand bookseller Dennis stumbles through a city still shaking off the war. While out procuring inventory, Dennis's life changes forever when he chances upon a novel that shouldn't exist; an entirely fictional book from inside another novel.

The book hails from The Great When, a magical shadow-London, where reality blurs with fiction and concepts like Crime, Poetry and Riot are incarnated as mystical beings that stalk the streets. This discovery brings terrible danger, as the book's presence leaves the doorway open between the two Londons, and The Great When must remain a secret.

Soon Dennis finds himself in the city's occult underbelly, negotiating sorcerers, gangsters, and murderers - some imaginary, some all too real, and all with plans of their own. If Dennis cannot return the book and close the gate, he risks dire repercussions...

Neither London will ever be the same again.