Richard's life is panning out just fine. He has a nice apartment, a good job, and a lovely, if not a little tight-wound, fiance. Sometimes, he thinks that there might be more to life than that, but he doesn't give it much thought until the night he and his fiance are to dine with his fiance's boss. On the way to the restaurant, a very dirty, very pretty, very injured girl falls bleeding onto the road in front of him. Against the wishes of his fiance he takes the girl home to care for her.
The girl's name is Door, and there are some suspicious looking people looking for her. Richard is entranced by this girl, whose mysterious and tragic life is so different to his own, and vows to help her.
His journey takes him to London underneath, a place where people who have fallen through the cracks of London above go. He gets Door safely away, and then tries to return home... only to find that he has become one of the unseen in his old life. His flat is relet, his work don't recognise him, and he has no fiance. He goes back to Door, the only person who can see him in the world, and joins her quest to avenge the death of her family.
Can Richard help to save London underneath? Can he get back to London above? And then... will he want to?