This book is about two boys, named Prosper and Bo who were separated when their parents died. Bo is sent to live with his aunt and uncle, and Prosper is sent to an orphanage. Prosper escapes from the orphanage, gets his brother and they run away to Venice. The boys find a girl named Hornet in Venice who takes them into the care of the Thief Lord, who is supposedly another runaway.

This book begins with the boys' aunt and uncle going to see the local detective, Victor. They ask him to find the boys, except he is reluctant, because he can feels that the aunt and uncle are not nice people. He gets this impression because of the way they talk about the boys.

While this is going on, Hornet and Bo are looking through the Venice town square. They are searching for any change or items...