Orphaned and unwilling to be separated, brothers Prosper and Boniface Hartleib run away to
Venice where they are taken in by a group of street children led by a proud orphan named Scipio, also known as "The Thief Lord." A man calling himself the Conte asks the "Thief Lord" to steal a wooden lion's wing for him. The runaway boys' rich but shallow aunt and uncle wish to adopt only sweet-faced Boniface. They hire a detective, Victor Getz, to find them. Over the course of his investigation, Victor has several run-ins with the gang. He discovers that Scipio is actually the son of a wealthy doctor, Dr. Massimo, but the doctor is emotionally abusive, so Scipio leads a double life on the streets. Victor finds their hideout, but is ambushed and tied up by the kids, who fear he will expose them. Victor distracts the gang by sending them to Dr. Massimo's house, where they learn Scipio's secret. In the meantime, Victor escapes, leaving a note saying he would not turn them in unless they went forward with stealing the wing. The wing belongs to a woman named Ida Spavento. At her house, Ida tells them the wing is from a magical
merry-go-round, which has the ability to change a person's age. She agrees to let them have the wing if they let her accompany them to the exchange. The gang, along with Ida, goes to make the exchange the next night, leaving Boniface and Caterina "Hornet" Grimani, the gang's only girl, to guard the hideout. When they return, they find Boniface and Hornet gone and discover the money they received for the wing is
counterfeit. With Victor's help, they find Hornet and Boniface. While the others stay with Ida Spavento, Prosper and Scipio return to the Conte's island home to get the money they are owed. On the island, the Conte's sister, Morosina, catches them climbing the wall and locks them inside the stables. The next morning, Scipio and Prosper meet the Conte and Morosina, who are now both young children. They tell him that the magical merry-go-round worked. They reveal that they were the penniless servants of the nobles who lived there, and schemed to repair the merry-go-round to reclaim childhoods that had been lost to hard labor. Scipio, who hates the vulnerability of being a child, requests a ride and comes off an adult. Just as Scipio gets off the merry-go-round, the gang's
fence Barbarossa arrives, intending to take his payment. As revenge for his cruel actions, Scipio (pretending to be his father) fools him into taking a ride. Barbarossa rapidly de-ages. He breaks the merry-go-round in panic, permanently stopping its magic, but he has become a young boy. Scipio and Prosper leave after promising the Conte that they will not talk about the merry-go-round. Barbarossa will be forced to give the Conte all the money in his shop safe. The next day, Prosper, Scipio, and Barbarossa meet up with the gang again. They do not recognize Scipio or Barbarossa and Prosper cannot explain. With Victor and Ida's help, Barbarossa is adopted by Prosper and Boniface's rich aunt Esther. Prosper and Boniface decide to live with Ida and go to school, along with Hornet. Esther catches Barbarossa stealing her jewelry and other possessions and sends him off to boarding school, where he becomes a menacing bully and takes over the title "Thief Lord". The adult Scipio works as Victor's assistant in his detective agency and is free from his father. ==Major characters==