When the parents of Prosper and his younger brother Bo pass away, their aunt Esther, portrayed by Carole Boyd, plans to adopt Bo while sending Prosper off to a boarding school. However, before this separation occurs, the resourceful Prosper decides to take Bo to Venice, a city filled with the enchanting stories their mother used to tell them. In the heart of Venice, they encounter a mysterious figure known as the Thief Lord, Rollo Weeks, who wears a mask and takes them to his secret lair—an abandoned cinema called the Stella. There, they meet other orphaned children whom Scipio has taken in: the clever Hornet, played by Alice Connor, the quick-witted Riccio, played by George MacKay, and the imaginative Mosca.
While this is happening, their aunt and uncle, Esther and Max Hartlieb, played by Bob Goody, travel to Venice to search for their missing nephews, enlisting the help of Victor Getz, portrayed by Jim Carter. As the plot unfolds, the Thief Lord receives an intriguing commission from a shadowy character known only as the Conte, requiring them to steal a wooden wing—an artifact from the bygone merry-go-round of the Merciful Sisters. Scipio, excited about the job, recruits the gang to help scout the mansion that guards this prized piece, but he soon disappears, leaving his friends puzzled and questioning his trustworthiness.
Amidst the chaos, it is revealed that Scipio is not the unfortunate orphan he pretended to be; rather, he is the son of the wealthy doctor, Dr. Massimo, portrayed by Robert Bathurst. When the gang attempts to execute their plan to steal the wing, they encounter Ida Spavento, played by Caroline Goodall, who, upon discovering their mission, agrees to assist them on the condition that she accompanies them. As they make their way to the convent of the Merciful Sisters, they uncover a remarkable secret: this enchanted merry-go-round has the power to turn children into adults and vice versa.
Scipio decides to take the gang to meet the Conte for the exchange, which proceeds smoothly at first. However, their luck takes a downturn when their hideout, the Stella, is raided by police, led there by Barbarossa, an antique dealer embodied by Alexei Sayle. This unfortunate event leads to the capture of Hornet and Bo, causing a stir among the remaining group, who mistakenly suspect Getz of selling them out. Despite the turmoil, Getz remains loyal, and he, along with Ida, successfully retrieves Hornet from the grasp of the police.
Later, Scipio attempts to persuade Prosper to accompany him in trying the merry-go-round, offering the chance to escape their childhood. Bo finds his way back to the Stella, only to be reunited with Getz, who brings him to safety. At this time, the Conte and his sister, the Contessa, unknowingly become children themselves after riding the merry-go-round. They then extend an unconventional offer to Scipio and Barbarossa in exchange for their valuable artifact. Scipio takes the chance and transitions into adulthood, while Barbarossa ends up a child.
The story culminates with Prosper reuniting with Bo, and Scipio embarks on a new adventure as Getz's partner in detective work. Their aunt and uncle, desperate to reclaim Bo, are confronted by Scipio, who bravely stands his ground. In a fitting conclusion, Riccio and Mosca share the rewards they earned from Barbarossa's stash amongst the group. With his share, Scipio purchases a boat for the boys and sets up bank accounts for them all. The final scenes evoke a sense of belonging as Victor, Ida, Prosper, Hornet, and Bo are seen aboard Scipio’s old boat, with Ida remarking that together, they are forming a wonderful new family. The essence of friendship, adventure, and the bonds of family resonate deeply as the credits roll, inviting viewers to reflect on the power of love and connection.
