Richard Mayhew is an ordinary young man working in London, with a fiancee, Jessica, a small flat and a life more-or-less figured out. By Jessica. He's not a particularly brave man, or imaginative, and Jessica has his life all sorted for him.
Everything in his life is turned upside-down - quite literally - when he stumbles across an injured girl on the footpath who asks for his help. Despite Jessica's insistence that he leave her there for someone else to take care of, he carries her back to his flat and at her behest, goes in search of someone called the Marquis de Carabas.
By involving himself in her life, her world, he becomes invisible in his own. After the girl, Door, has gone back to London Below, Richard finds that no one can see him or hear him, that they've removed his desk at work and are renting out his apartment while he's in the bath. Jessica can't remember his name. The only thing left for him to do is seek out Door in London Below and somehow get his life back.
The world of London Below is vivid, labrynthine, confusing, other-worldly, bizarre and smelly. The sewers, the Underground (the Tube) and a vast plane of tunnels, caverns and bunkers form curious homes for curious creatures, most of which Richard has trouble believing are real. To say he's a bit slow would be an understatement.
He gets caught up in her mission to find out why her family was murdered and who wants her dead. Door has a unique talent - she can open any door, can create doors where they weren't before, and someone wants this skill badly enough to send two violent things after her, called Mister Croup and Mister Vandemar. She enlists the aid of a bodyguard, the famous Hunter, and goes on a quest to find the Angel Islington.