Jessica's discovered what her Midnight gift is, and is getting to know her fellow Midnighters better: Rex, the seer; Dess, a polymath; her boyfriend Jonathon, who can fly in the midnight hour; and Melissa, for whom that hour is blessed relief from the rest of the town's thoughts. The stakes are rising, as the darklings plans against them begin to take form, and it appears as though they're being aided by people, though how that's possible when only Midnighters can move in the folded hour, and darklings only exist then, the teens are unsure.
When Dess begins to more closely plot the edges of the blue zone - the area affected by the midnight hour - she notices some interesting anomalies, and tracks down a woman in an apparently deserted house. Madeline is the only survivor of what used to be a cluster of midnighters, all wiped out forty-nine years ago in a lightening purge. Is she everything she seems? What is she hiding? And why are the darklings after Rex?
Maths is tightly wound up in the Midnighters universe - the ancient darklings fear new things (like non-elemental metals) and the number thirteen, but twelve is safe for them; dates and multiplications of both numbers hold significance, and the emphasis on polymath Dess and on maths (explicitly dissociated from numerology) increases in this chapter of the trilogy. Weapons uniquely named with a thirteen-lettered word (the words lose power after use) are more deadly, with additional names adding power, resulting in such delightful creations as Categorically Unjustifiable Appropriation and Brogdignagian Perambulation.
In addition, this book begins to explore bigger issues, like what happens when you have the ability to not just read minds but to influence them? When you have powers others don't, how do you regulate yourselves? And - little sisters: how much of a pain are they?
This is a worthy successor to the first in the series, and I was glad I held off reading it until I tracked down the final, as I had to move straight onto that. - Alex