The girls follow it into the basement of a building, where they are drawn through a gap between the worlds of London and Un Lun Dun (or UnLondon).

In conversation he realises that Zanna is the "Shwazzy", a prophesied chosen one who is destined to save UnLondon from the Smog – an evil, sentient cloud of pollution.

Brokkenbroll, master of broken umbrellas (or unbrellas) arrives in time to shoo the Smog away using a new technique he and Benjamin Unstible (a Propheseer who was presumed dead) came up with.

Instead, the city turns to Brokkenbroll and Unstible who begin handing out Smog-resistant unbrellas to defend the people of UnLondon.

However, Deeba, who still remembers UnLondon, begins to search for things related to it on the internet, hoping she can find someone to talk to.

The Book, although it has proved to be less-than-accurate, agrees to help them fulfil the Shwazzy's tasks and defeat the Smog in the limited time Deeba has before everyone in London forgets that she exists (this happens to everything that comes to UnLondon).

Deeba and Hemi are aided in their quest by Obaday, Rosa, Jones; the utterlings Diss, Bling and Cauldron; and Yorick Cavea and Curdle the milk carton.

Mieville's acknowledgements section mentions that the book is influenced by Lewis Carroll (author of Alice in Wonderland), and particularly Neil Gaiman for Neverwhere.