Richard is on the way to a dinner with his fiancée, Jessica (not "Jess") when a young girl falls to the sidewalk in front of them, obviously hurt. Despite Jessica's threats of ending the engagement, Richard feels he must help. The girl, named Door, asks not to be taken to a doctor or hospital so Richard brings her back to his flat where she recovers. Because of this act of kindness, Richard is forced to endure visits from the vile Mr. Croup and Mr. Vandemar, to deal with the roguish Marquis De Carabas, and to converse with a rat. Worse still, his friends and co-workers suddenly don't seem to recognize him.
Richard is seeking Door in London Below, trying to find a way to get his normal life back. Meanwhile, Door and De Carabas search her home for clues to the murder of her family. A message in her father's journal tells them to seek the Angel Islington, but first they decide to hire her a bodyguard at the Floating Market. On the way to find them, Richard must cross Night's Bridge where he meets an enigmatic woman and faces the fear of the Night. Croup and Vandemar plot to have a man under their employ win the position of Door's bodyguard.
Richard, Door, Hunter, and De Carabas go to Earl's Court (a subway car, seen and used only by the denizens of London Below) where the Earl kicks De Carabas out of his "court" and then tells the rest of them how to find Islington. Croup and Vandemar waylay Richard and Door on the way there (on orders from their unseen client to scare them) and tell them that there is a traitor in their midst. They escape and run into the British Museum where they must find the Angelis in order to get to Islington. De Carabas makes an unexpected housecall.
De Carabas concludes his business with Croup and Vandemar. Richard and Door speak with the Angel Islington who sends them on a quest for a key which is held by The Blackfriars. They must escape the lair of Serpentine, where Hunter receives a spear to hunt The Beast of London. The Blackfriars set an ordeal for Richard to pass through before he may obtain the key. The identity of Croup and Vandemar's employer is revealed.
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. The traitor is revealed.
Richard and company must traverse the labyrinth wherein dwells The Beast of London. The truth behind what has been happening is revealed and Croup and Vandemar are faced at last. The key is used. Richard must make an important decision.
