Join Kiki and the Irregulars in another underworld - and underground - adventure. With Oona taking the lead this time, the Irregulars band together to protect New York City and its secret subterranean world, doing battle along the way with Manhattan's gangsters, rodents, diamond merchants, society figures, and assorted forces of evil. Some of the Irregulars must come to terms with deadly secrets from their own pasts, and each of the girls finds her unusual skills and her friendships put to the ultimate test.