Brothers Prosper and Boniface have to live with a mean aunt and uncle after their parents' deaths. When they learn that their aunt wants to separate them and just keep young, cute Boniface they run away to Venice. Once there they are living on the streets until they meet a group of orphans who live in an abandoned theater. The orphans are taken care of by another boy named Scipio, the Thief Lord, who steals things of great value and sells them to provide for all of them. The brothers learn that their aunt has tracked them to Venice and has hired a private investigator to locate them. So with the help of their new family they plan a way to carry out the biggest theft ever and throw the detective off their trail.