Two girls travel to a strange mirror world and help its citizens fight off an evil smog monster. Zanna Moon and Deeba Resha are playing when they see a fox, but strangely it doesn't seem scared of them at all. This is just the latest in a series of strange events and encounters that Zanna has experienced recently.
Zanna goes over to spend the night at Deeba's house and in the middle of the night they hear a commotion which turns out to be a broken umbrella moving all on its own. The girls follow it and find a door that leads them to UnLondon, a surreal version of the city they live in.
They meet a strange cast of characters, including a half ghost boy named Hemi, and soon Zanna is recognized by the Unlondoners as the "shwazzy", the prophesied Chosen One who will save them from the evil Smog. The Smog is a sentient dark cloud that can possess people and turn them into smog-zombies. The Smog was banished from regular London by something called the Klinneract. Unlondon is what is known as an 'abcity' and everything there is either a discarded or obsolete thing from our world, and things that cross over are eventually forgotten by regular London.
The Shwazzy's coming has been predicted by the Propheseers who are the main rulers of the city, and by their talking Book that contains the clues to defeating the Smog. The girls are skeptical but Zanna decides to take up the challenge, however she fails to defeat the Smog and is nearly killed when she inhales some of the smog. Fortunately the Smog is fought off temporarily by Brokkenbroll, the master of broken umbrellas, and a Propheseer named Unstible. Brokkenbroll has covered his umbrellas with a special chemical compound that seems to hold off the Smog, and he distributes them to the citizens.
Ill from the Smog, Zanna has her memories of Unlondon erased and she and Deeba return to their world. But Deeba can't stop thinking about Unlondon and after she searches online for information she finds out that Unstible was declared dead in our world while studying the Clean Air Act that cleaned up London. Suspicious, she decides to return.
Back in Unlondon she enlists Hemi's help and they confirm from the ghost world that Unstible is indeed deceased. They warn the Propheseers but Brokkenbroll captures them. It turns out he is working with the Smog, Unstible was a smog-zombie all along. Even worse, his umbrellas are no real protection.
Eventually they escape and warn the Propheseers, but they are devoted to the prophecy as it is written in the Book and therefore ignore Deeba. The kids steal the Book and use it to work through the prophecy anyway without Zanna the Chosen One. It is a race against time, though, since if Deeba stays too long she will be forgotten at home and unable to return.
Working with a motley crew of other citizens of Unlondon Deeba takes on the mantle of the Unchosen One, and manages to defeat Brokkenbroll and the Smog by using the UnGun, which creates a vacuum rather than firing anything and is therefore the best weapon against the Smog.
With the Smog finally defeated Deeba is given the choice of staying in Unlondon if she wishes to. Puzzled she reminds them that visiting is as easy as stepping through a portal, and she's already done so twice. She will go home but visit often.
Best part of story, including ending:
It's a very clever subversion of the usual Chosen One story, and the language is full of playful puns.
Best scene in story:
I really liked when Deeba pays her way into Unlondon with "new" words that are transformed into strange creatures after being spoke out loud.
Opinion about the main character:
Deeba is the funny sidekick character and is suddenly thrust into the spotlight when the "protagonist" turns out not to be so Chosen after all, this makes her both funny and heroic, which is nice.