Life becomes more interesting for Ananka Fishbein when, at the age of twelve, she discovers an underground room in the park across from her New York City apartment and meets a mysterious girl called Kiki Strike who claims that she, too, wants to explore the subterranean world.

While Kiki Strike is in Paris trying to stop her evil cousin, the princess Sidonia, from all sorts of terrible deeds, it is up to Ananka and the other Irregulars help Kiki find the cure for baldness, foil the evil plans of Oona's twin, and keep Ananka herself from falling in love with wrong young man.