Richard Mayhew leads a normal life in London. One day he sees the girl Door injured on the sidewalk. He helps her, but because of this he ceases to exist for normal Londoners and becomes part of London Below. Here he joins Door, Hunter and Marquis de Carabas in search of The Angel Islington who knows more about the murder of Door's family and who may also know a way for Richard to get back to London Above. While searching for the answers, they are chased by Messrs Croup and Vandemar.