The Thief Lord concerns two brothers, Prosper and Boniface (Bo), who run away to Venice, Italy after their mother dies. After they are orphaned, Bo (who is five years old) is sent to live with his cruel aunt Esther while Prosper (who is around twelve) is sent to a boarding school. However, Prosper runs away with Bo. Together, they decide to go to Venice, the magical city that their mother always talked about. The boys are soon be friended by four other orphans: Hornet, Riccio, Mosca, and their mysterious leader, Scipio (or the "Thief Lord") The children take Prosper and Bo to an abandoned movie theatre, the Stella or Star-Palace, which is where they all live. The children survive by selling the things that Scipio steals from wealthy houses to an old shopkeeper, Ernesto Barbarossa. All is well until Prosper and Bo's aunt Esther soon finds out that the boys are in Venice. She contacts a disguise-loving detective, Victor Getz, to capture the boys. Meanwhile, Barbarossa tells the children about one of his clients that needs someone to procure (steal) something for him. The Thief Lord accepts the job and meets the client, who is called The Conte, in a confessional with the others. They find out that the object they are to steal is a wooden wing and that they are to steal it from a wealthy photographer's house, whose name is Ida Spavento. If they manage to steal it, they will receive five million lire.
While Mosca, Prosper, and the Thief Lord are inside the confessional talking to the Conte, Riccio, Hornet, and Bo wait outside. Bo is distracted by a tourist who is actually Victor in disguise. Victor persuades Bo to tell him his name and that he lives in an abandoned movie theatre which was a very bad choice for them. When the others see Bo talking to Victor, Victor escapes, but watches the kids as they enter a store. The older children find out that Victor "the tourist" is actually a detective (which is why they enter the store.) Hornet follows Victor and creates a distraction by pretending that Victor was trying to kidnap and hurt her while the others escape back to the movie theatre. Hornet manages to steal Victor's wallet and the children find out that Victor is a detective and is trying to capture Prosper and Bo.
Victor visits the very wealthy Dottore Massimo, the owner of the Stella, and finds out that Scipio the "Thief Lord" is actually Dottore Massimo's son, instead of an orphan like he has told the other children. Scipio manages to warn the children that Victor is coming. They capture Victor when he comes looking for them and hold him prisoner. However, Victor tells them that Scipio is actually a young rich boy. When the group doesn't believe him, he tells them to visit Doctor Massimo's house. They go to the house and find out that Scipio has lied to them all along and actually stole all the loot from his own father. When the orphans come back to the movie theatre,Riccio was the maddest and all the kids were feeling betrayed by the Thief Lord, they discover that Victor has escaped. However, he leaves a note telling them that he will not tell Esther where Prosper and Bo are, if they do not continue the heist to steal the wing. The orphans decide to sneak into Ida's house to steal the valuable wing for the Conte without Scipio. However, when at the house they find out that Scipio is already there. They are all eventually caught and Ida reveals the story of the wing, which was actually part of a magical merry-go-round that can turn a child into an adult and an adult into a child. The group decides to give the wing to the Conte. On the night of the trade, Hornet and Bo stay at the Stella while the others trade the wing in. After they make the trade with the Conte and his sister, the Contessa, they follow behind on Ida's boat where they find the Conte's island, where he must be keeping the merry-go-round. Meanwhile, Hornet and Bo are captured by the police. Hornet is taken to an orphanage while Bo is returned to Esther.
The children find out where Hornet and Bo are and that the money they received from the Conte was fake. Prosper is in despair, and decides to leave with Scipio to go to the island. Meanwhile, Bo is with his aunt and uncle, where he throws tantrum after tantrum, eventually running away. Victor finds him at the Stella and takes him to Ida's home to find that Prosper is not there. Meanwhile, Prosper and Scipio reach the island. They meet a younger version of the Conte and Contessa, and Scipio demands a ride on the merry-go-round, so he can grow up, and move out of his home and away from his father. He successfully manages to age correctly, but Barbarossa shows up and demands to see the Conte's treasure. They trick him into riding the merry-go-round and he is turned into a small child. In an attempt to stop, he breaks the wing, effectively rendering the Merry-Go-Round useless, and making the age changes irreversible. Soon Scipio and Prosper return to Ida's with baby Barbarossa, where Prosper learns that Bo is no longer wanted by their aunt and can live with Prosper. Meanwhile, Hornet is taken out of the orphanage by Ida, who pretends to be her godmother.
Soon afterwards, , Prosper, and Bo decide to live with Ida and go to school while Mosca and Riccio decide to live with Hornet in an abandoned warehouse (since they do not want to go to school). Scipio works for Victor and sends letters to his father saying that he is alright, but can't come home. They all live happily and visit each other often. Barbarossa has tricked Esther into adopting him when he finds out that she is rich. He acts sweet and innocent to get her to take him in, but when he arrives at Esther's house he is caught stealing. Tearfully, Esther sends him to a boarding school where he is the biggest bully, making children do his homework, polish his shoes, and sometimes even steal things. He also makes the children call him a famous name said throughout the book... "The Thief Lord".All went well; without the greedy Barbarrosa.