Morrigan Crow and her friends have survived their first year as proud scholars of the elite Wundrous Society, helped bring down the nefarious Ghastly Market, and proven themselves loyal to Unit 919. Now Morrigan faces a new, exciting challenge: to master the mysterious Wretched Arts, and control the power that threatens to consume her.

But a strange and frightening illness has taken hold of Nevermoor, turning infected Wunimals into mindless, vicious Unnimals on the hunt. As victims of the Hollowpox multiply, panic spreads. And with the city she loves in a state of fear, Morrigan quickly realizes it's up to her to find a cure for the Hollowpox, even if it will put her - and everyone in Nevermoor - in more danger than she ever imagined.