The dark was deep; it opened like a cavernous mouth and swallowed the boy whole.

Cassia Thorne leads a double life. By day, she sells ballads at Bartholomew Fair. By night she spends her evenings locked up in Fleet Prison.

Cassia has plans of escaping both of these lives. But this year there are rumours of children being snatched off of the streets of London and no one willing to help. So Cassia decides to take matters into her own hands; with the help of a young pickpocket, Teo and her friend Felix, Cassia starts to investigate the disappearances.

She soon discovers a sinister conspiracy at the heart of the city – mysterious men in blue coats and whispers of a beast that lives in the deep dark tunnels below the city. Can Cassia get to the bottom of the mystery and rescue the missing children before it’s too late?