The Thief Lord is about two orphaned brothers, Prosper and Boniface (referred to as Bo after his first introduction), who have run away to Venice because their aunt, Esther Hartlieb, and her husband wish to adopt angel-faced five-year-old Bo, but not twelve-year-old Prosper. When the novel begins they have been taken under the wing of Scipio, an apparently daring youthful cat burglar who likes to be known as the Thief Lord and who wears an odd beaked mask (Doctor Death Mask), and his little gang of street children, Mosca, a member who's father left on a ship and hasn't been seen since, Riccio, a spiky-haired pickpocket, and Hornet, the only girl in the gang. Scipio has found the children a hideout in a disused movie theater which he calls the Star Palace (Stella). Though he does not stay there himself, he often visits, bringing the children loot they sell to a crooked antiques dealer named Ernesto Barbarossa. Prosper turns out to be talented at negotiating with this villain, making him pay four times his original offer for a pair of sugar tongs, and giving Prosper and Bo permanent positions in the gang.
However, Esther has figured out that the runaway boys have fled to Venice, since their late mother loved the city and often told them magical stories about it. Esther hires a kind and lonely Private detective, named Victor Getz, to find the boys. Meanwhile, Barbarossa has offered the Thief Lord a commission; to steal something for a mysterious Venetian known only as the Conte. Scipio meets the Conte in a confessional in the Basilica. He goes into the Cathedral with Prosper and Mosca, and learn their objective: to steal a wooden wing from a woman named Ida Spavento. Bo is left outside in St. Mark's Square with Riccio and Hornet, where Bo is spotted by Victor. The detective manages to talk to Bo, and, in his childish innocence, Bo divulges that he lives in a disused cinema. Victor's investigation of this lead reveals far more than the reader expects and shatters the children's fragile attempt at surviving in the adult world, whilst their commission from the Conte leads them and the novel into a magical and fantastical ending.
The group soon discovers that the "Thief Lord" was actually the son of a wealthy man, and had stolen all of his items from his father. The group feels betrayed to know that Scipio was not who they believed. While the orphans stalk Ida's house and prepare for the theft, Victor finds the location of the Star Palace. He is then captured and becomes a prisoner to the gang, and becomes friends with Mosca and Bo. After the detective escapes and threatens to snitch if the children continue their heist, the orphans still sneak in to Ida's house to find the valuable wing for the Conte. Instead, they are caught and Ida reveals the story of the wing, which was actually part of a magical merry-go-round that can change a person's age. The group decides to team up with Ida, and 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. Meanwhile, Hornet and Bo are found out by police. Hornet is taken to an orphanage while Bo is returned to Esther.