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 of the Accomplished Wundersmith, and control the power that threatens to consume her.

Meanwhile, 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. There are whispers - growing louder every day - that this catastrophe can only be the work of the Wundersmith, Ezra Squall.

But inside the walls of Wunsoc, everyone knows there is a new Wundersmith - one who’s much closer to home. With Nevermoor in a state of fear and the truth about Morrigan threatening to get out, the city she loves becomes the most perilous place in the world. Morrigan must try to find a cure for the Hollowpox, but it will put her - and everyone in Nevermoor - in more danger than she could have imagined.