When Richard goes to London to work, a strange old woman predicts he will go a lot farther -- and it will involve doors.
In London itself, Richard falls into the orbit of Jessica, who thinks he can be made over into a proper husband, and Door is fleeing assassins who murdered the entire rest of her family by opening doors. At a crucial moment, when Richard is trying to appease Jessica, Door opens a door to someone who will help her. Richard, seeing that she's hurt, carries her back to his apartment.
The assassins, Mister Croup and Mister Vandemar, try to catch her there. They open the bathroom door but she's not there -- in spite of her having been moments before.
After some messages sent by pigeon and rat, Richard is set off to meet the marquis of Carabas, who comes and takes Door, and they assure him that they're leaving him there, in London Above, where he belongs, and going to London Below.
Except that his life starts to vanish about him, with no one even able to see him, and Richard finds that he has fallen through the cracks to London Below. He tries to find Door.
And the subsequent story involves the Floating Market, a bodyguard, an enormous wild boar, peasoup fog, a girl whose name Richard did not quite catch -- something like Anaesthesia -- a key, an ordeal, an earl, and an angel.