Cornelia Funke's The Thief Lord is set in Venice, Italy, in autumn. Twelve-year-old Prosper and five-year-old Boniface are brothers being pursued by Victor Getz, a small-time private detective. The boys’ rich aunt and uncle, Ester and Max Hartlieb, want to find Bonice (called "Bo" throughout the novel) to adopt him because the boys’ mother has passed away. However, they only want to adopt the angelic-looking Bo; they intend to send Prospero to a boarding school because they believe that Prosper is too old to adopt.
Wanting to stay together, the boys run away. Their mother used to tell them fairy tales about Venice, the city to which they are now drawn. Once they arrive, the brothers meet Caterina, a slender young girl nicknamed "Hornet" for her long, skinny braid. Homeless, they stay at the Star Palace, an abandoned movie theater, with Riccio, a boy about Prosper’s age, and Mosca, a strong boy of African descent. Thirteen-year-old Scipio Massimo, "the Thief Lord," takes care of the group at the Star Palace, wears a mask, and pretends to be an orphan too.
The group survives by selling stolen items to Ernesto Barbarossa, or "Redbeard," a small-time antique dealer who enjoys swindling people. Redbeard introduces the group to the Conte, who hires Scipio and his charges to steal a treasured wooden wing from Ida Spavento for five million lira. Before they can do so, Victor finds the brothers, but is captured by the children at the Star Palace. He had earlier visited Scipio’s father, Dottore Massimo, and discovered that the Thief Lord is actually a very wealthy young man. Victor tells the group about Scipio, and they learn that all of the stolen items sold to Redbeard are actually from Scipio's own home.
The group still decides to steal the wing, but they are caught in the act by Ida. A former orphan, Ida reveals that the wing was part of a magical merry-go-round. It can turn a child into an adult and an adult into a child, depending on the way the carousel spins. Ida decides to help them take the wing to the Conte. Then, the group goes to the island with the merry-go-round because they discover the Conte's money was counterfeit. The Conte and his sister are younger when they arrive. Scipio takes a ride to become older because he hates being told what to do by his wealthy father. Without warning, Barbarossa arrives and wants to see the Conte’s treasure. They trick him into taking a ride, and he breaks the carousel after he is turned into a small child. All of the age changes are now permanent.
Most of the children end up living with Ida. Riccio and Mosca do not want to go to school, so they continue living in the theater. Barbossa is adopted by Ester, and Scipio works for Victor. Even as a child, Barbossa cannot stop his conniving and stealing tendencies, so Ester sends him to a boarding school where he becomes a bully.
The Thief Lord was originally published in Germany in 2000, and was later translated to English and published in 2002 by The Chicken House publishing company. The novel has won the 2003 Mildred L. Batchelder Award, the 2002 New York Times Notable Book Award, and the 2005 Young Readers Choice Award (Senior Division).