Prosper and Bo are orphans on the run for their cruel aunt and uncle. The brothers decide to hide out in Venice, where they meet a mysterious thirteen-year-old boy who calls himself "the Thief Lord." Brilliant and charismatic, the Thief Lord leads a ring of street children who dabble in petty crimes. Prosper and Bo delight in being a part of this colorful new family and then the Thief Lord invites them on a magical adventure to a land of forgotten mysteries that will change their lives forever.