There is the London we know. The London of hardworking, white-collar Englishmen, tourists, the workings of parliament.

But there is another London. One made up of the things and people that didn't quite mesh with their London, of whatever time, and fell through the cracks.

Richard Mayhew is a very normal man. He works fairly hard and fairly well at his office. He's engaged to a demanding but gorgeous woman, Jessica, determined to bring out his "potential." The oddest thing about him, really, is that he collects those plastic trolls with bright fake hair, and that began by accident.

But one day Richard sees a girl. Other people don't. Jessica barely notices her. But Richard sees her. She's bleeding. She needs help. He gives it to her.

But helping Door (no, it's not short for anything), turns out to get Richard in way over his head. Talking with pigeons and rats. A man who's called the Marquis de Carabas. People living camped out on London roofs. And some very suspicious men asking for her.

Worse is when she leaves. Because Richard has fallen through the cracks now. To his London, he's ceased to exist. Taxis nearly run him over when he hails them. A realtor gives a sales pitch for his apartment while he's in it. Only with much effort does he get Jessica to remember his name.

But there's a whole new group of people that he does exist to. Including Door and the Marquis. So seeks them out. And becomes truly involved with Door's quest to find those responsible for killing her family.

Welcome to London Below. While you're here, mind the gap.