When Richard Mayhew helps a wounded girl on the street, he finds himself beginning a nightmare journey that plunges him into a weird parallel world underneath London. The girl is on the run from two cut-throats who she thinks killed her family. But why are they after her? And now that Richard is involved, will he ever return to a normal life?

Richard Mayhew has fallen into a parallel world that exists beneath the streets of London and he wants his old life back. His only chance is to find Door and the Marquis, who have set off for the Floating Market in search of a bodyguard to protect Door from Croup and Vandemar. But Richard could do with protection himself.

Angel Islington is the only person who can tell Door who killed her family and help get Richard back to his old life. But how do they get to him? The Marquis suggests a visit to Earl's Court - but it has a worrying tendency to move around. Meanwhile, Croup and Vandemar alert Door to the fact that there is a traitor in her midst... but who?

Our band of heroes attends another floating market. Door meets an old friend and Richard makes a new one. Old Bailey must make good on his debt. Richard, Door, and Hunter seek Downstreet in an attempt to find their way back to Islington.

With the sacred key in his possession, only one person stands in the way between the Angel and his ultimate ambition - Door. As the apocalyptic climax looms, everyone faces their own destinies.