Zanna, age 12, and her friend Deeba, start noticing strange things happening that all center around Zanna, and one night they go exploring (they literally go underground, into a basement at one point) and end up in a different world entirely: Un Lun Dun (i.e., Un-London), which is where all the lost or misfit things and people of London end up. They learn that Zanna is believed to be the Chosen One (just as Girls Underground are often revealed to have special destinies) who will defeat a terrifying enemy: the Smog (and his many minions). They quickly meet a bunch of interesting characters that become their companions for varying amounts of time. Then Zanna confronts the Smog, and is hurt, and Deeba saves her and brings her back to their world (as many GU protagonists briefly return home), which commences a chapter quite poignantly titled “The Powerful Resurgence of the Everyday.” Deeba finds that their parents have not noticed their absence at all (an extreme example of oblivious or neglectful GU parents) – and Zanna herself has no memory of the events.
SPOILERS It eventually becomes clear that it is Deeba who is the real heroine (a fascinating turn of events which I have not encountered yet in any other example). She discovers a fraud perpetrated by the adversary, and returns to UnLondon to help (note the second, deliberate journey to the otherworld, much like Alice first stumbles into Wonderland but later ventures into the looking glass of her own free will). She meets up with her old companions and acquires some new ones too. They go on an elaborate quest together. She is fighting against time, because in just a few days everyone from home will forget her completely, and the Smog is mounting a terrible attack at the same time. After being betrayed by one of her companions, she faces the Smog and defeats it, and is able to return home for good.
“When she was there, she had wanted desperately to come home. Now, even though she was truly happy to be back, she was wistful that she could never say anything about the most amazing place she had ever been.”