Richard Mayhew leads a boring life in London with his shrewish fiancee until one night Door, a magical girl from the world underneath London, appears in front of him, begging for help. Richard helps her, and for his troubles loses his fiancee, his job, the ability to be seen by anyone around him, and the ire of two killer-for-hire, Mr. Croup and Mr. Vandermar.
Door asks him to fetch another person from the London Underground, the Marquis de Carabas, to help her get home. The Marquis, Door, and eventually Richard all make it underground. Door discovers that her family was killed on someone else's orders, and hires a bodyguard, Hunter.
The assembled group head for the one person who Door's Father said they could trust, the Angel Islington. Islington says that he will help them, but they will need to get a key with the ability to open any door, which is held by the Black Friars.
Richard manages to retrieve the key, and Door secretly has a copy made while at one of the floating markets. On their way back to the Angel, Door is captured by Croup and Vandermar, along with Hunter, who had been working for them all along. Hunter is killed by the Beast of London, who is dispatched by Richard, earning the title of the greatest Hunter in London Below. Richard and the Marquis then chase after Door.
Door is being held by the Angel Islington, who is angry about being thrown out of Heaven for allowing the destruction of Atlantis. He intends to open the door to anywhere, and use it to attack and conquer Heaven. Door, however, uses the fake key she had made, and sends the Angel, along with Croup and Vandermar into the futhest reaches of space, where they will never come back.
In return for all his help, Richard is allowed to go back to his normal life. But once he gets back, he discovers that he liked being in London Below even more. He pleads with Door to come and get him, scratching a door into a brick wall with Hunter's knife. The Marquis appears through the door, asks him if he is coming, and the pair of them go back into London Below.
Note: This skipped a lot, including the murder and resurrection of the Marquis. Also, this is one of the strange cases in which the movie (in this case, miniseries) was around before the novel. The novel is now allegedly being made into a big screen movie.
Thanks Paul B!