In this period crime mystery, Cassia Thorne begins investigating as children start disappearing from the streets of London.

'In the deep of the dark, a pair of eyes watched. A great mouth opened wide…'

Cassia Thorne leads a double life. In one she is a respectable young music teacher for the children of the elite. In another, she sells ballads on the streets and spends her evenings locked up in Fleet Prison. When the Bartholomew Fair comes to town, it brings joy to the city but also rumours of children going missing off the streets of London. With the help of a young pickpocket, Teo and her friend Felix, Cassia starts to investigate these disappearances. She soon discovers a darkness at the heart of the city and a beast that lives in the deep, dark tunnels under it. Can Cassia save the children when the conspiracy goes all the way to the top?