Fin is a foundling growing up in the Hackney marshes in Victorian London, living in a tiny attic and selling leeches for a living. When she accidentally discovers she is the lost-lost child of a rich Polish family called the Kaminskis, she is swept up into a supernatural adventure where she has to use everything she has learned on the streets of London and deep in the marshes to stay alive. For the Kaminskis are haunted by an ancient evil - and Fin is the key to stopping it forever . . .