It’s June 15, 1767. In the Italian village of Ombrosa, the di Rondò family—Baron Arminio, his illegitimate brother the cavalier avvocato, and his wife the Generalessa; their children, Battista, Biagio, and Cosimo; and the boys’ tutor, Abbé Fauchelafleur—sit at the lunch table, where Cosimo refuses to eat his snails. When Cosimo turned 12, he and eight-year-old Biagio began eating at the adults’ table, ending a period of mealtime mischief. Since then, Cosimo has been rebelling against his parents. Refusing to eat the snails is a grave offense, so Cosimo runs from the table and climbs into a tree. He declares that he’s never coming down and climbs into the garden next door, which is owned by Baron Arminio’s rivals, the D’Ondariva family. In the D’Ondarivas’ garden, Cosimo finds a beautiful young girl swinging. The girl, Viola, taunts Cosimo, but they discuss rules for a game in which Cosimo cannot come down from the trees. When Viola’s aunt appears and invites Cosimo to have hot chocolate, Cosimo, prideful, refuses and climbs away. Back in his own garden, Cosimo announces to Biagio that he’s not coming down. This disturbs Biagio, but he does as Cosimo asks and brings him a blanket and other supplies.
Cosimo wakes the next morning and goes in search of the fruit thieves, a band of local boys who steal farmers’ fruit. They taunt Cosimo for being wealthy and mention a girl they call the Sinforosa. Their laughter alerts the farmers to their whereabouts, but Cosimo leads them to safety through the treetops. Gradually, Cosimo learns that the Sinforosa is Viola, and she used to protect the fruit thieves—until she betrayed them and caused them to get caught. Meanwhile, Cosimo desperately wants to impress Viola. When he and the thieves hear her hunting horn one day, they race to her, the boys on the ground and Cosimo in the treetops. Cosimo blurts out that he hasn’t left the trees and the children begin a game of chase. Viola seems to lose interest when the fruit thieves abandon the game, and Cosimo suspects she started the game to make all the boys angry. Around this time, the Generalessa begins watching Cosimo through a field telescope and is the first in his family to accept that he’s not coming down.
For his first few days, Cosimo explores the treetops. After a while, Baron Arminio sends the cavalier avvocato to the D’Ondarivas’ garden to capture Cosimo. This attempt is unsuccessful and only drives Cosimo to try harder to impress Viola. To do this, Cosimo ventures deep into the woods, where he comes upon a vicious wild cat and manages to slay it. He carries the cat back to show Viola, but she shows little interest as she climbs into a carriage to leave for school. Cosimo is heartbroken, but he makes a fur hat out of the cat’s pelt. Baron Arminio spends his time worrying that his claim to the local dukedom is ruined now that his heir lives in the trees, a worry that Biagio suggests was silly to begin with—Baron Arminio will never get the duchy. One day, Baron Arminio finds Cosimo and they discuss the possible, unforeseen consequences of Cosimo’s rebellion. The next day, Cosimo resumes his lessons with the Abbé Fauchelafleur, but from the trees. From this point onward, things are normal between Cosimo and his family, aside from Cosimo’s choice of living quarters. Baron Arminio tries to keep Cosimo’s rebellion a secret, but this fails when the Count d’Estomac visits. The Count finds Cosimo delightful and promises to spread word of Cosimo throughout the courts of Europe. The young Count d’Estomac, meanwhile, surprisingly gets engaged to Battista.
Soon Cosimo knows how to live and work in the trees, but he needs a hunting dog. One day, he sees a dachshund puppy tagging along with some hounds. When the hunters insist the dachshund isn’t theirs, Cosimo encourages the puppy to raise a fox, adopts the puppy, and names him Ottimo Massimo. He discovers that the puppy belonged to Viola, so he carves his name, Viola’s name, and Ottimo Massimo’s name in a tree in Viola’s garden. For the rest of Cosimo’s adolescence, he essentially runs wild. Cosimo gets to know the cavalier avvocato and begins to reevaluate his dislike and distrust for him when Cosimo learns his uncle keeps bees, but in a way to escape Baron Arminio’s notice. The cavalier avvocato sometimes talks to Cosimo about aqueduct systems, but their talks don’t go anywhere in practice. Mostly, Cosimo takes his uncle as an example of what not to be: separated from society.
Rumors circulate that the bandit Gian dei Brughi is on the loose, but as Cosimo listens to itinerant charcoal burners, he gradually comes to suspect that the bandit isn’t someone to fear. Thus, when the bandit himself runs under Cosimo’s tree to escape capture one day, Cosimo helps dei Brughi. He discovers that dei Brughi loves to read novels and begins ferrying books to him. In Cosimo, this ignites a lifelong love of reading and learning. For dei Brughi, however, this leads to his demise: his love of books makes him a poor thief, and constables capture him. Cosimo reads to dei Brughi through the bandit’s cell window, and in the moments before dei Brughi hangs, Cosimo tells him the ending of their current book. Following dei Brughi’s death, Cosimo begins seeking the Abbé Fauchelafleur out for lessons, but Cosimo is often the one to act as the teacher and so introduces his tutor to Enlightenment ideas. This results in the Abbé’s arrest, as those ideas are considered heretical. The Abbé’s arrest doesn’t stop Cosimo, however; he begins writing to the great minds of the era and purchases the entire set of Diderot and D’Alembert’s Encyclopedia.
Over the dry summer, arsonists set fire to the woods. Cosimo dedicates himself to starting a fire brigade and proves himself a skilled and charismatic leader. On Cosimo’s 18th birthday, Baron Arminio seeks Cosimo out, gives him his sword, and reminds him of his duty to the title of Baron di Rondò. Around this time, Cosimo also notices the cavalier avvocato behaving strangely. He eventually learns that his uncle is in cahoots with Turkish pirates and is helping them rob ships. When Cosimo discovers the pirates hiding loot in a cave, he leads the hungry charcoal burners in fighting the pirates so the charcoal burners can eat the foodstuffs. Cosimo finds himself on the mast of a boat with Ottimo Massimo and the cavalier avvocato. The cavalier avvocato mutters the name Zaira and when they come across a pirate ship, the pirates kill the cavalier avvocato. Cosimo later tells the story to make his uncle’s death honorable and glorious to try to save Baron Arminio from grief, but Baron Arminio dies of depression not long after. Cosimo is now the Baron di Rondò, but little changes in his life.
When Cosimo learns of a group of Spaniards living in the trees of Olivabassa, a nearby city, he travels there to meet them. A Jesuit named Father Sulpicio introduces Cosimo to the leader of the nobles, Frederico Alonso Sanchez (Don Frederico), and a man named El Conde. King Carlos III banished these nobles, and now they wait for an invitation home. Cosimo soon falls in love with Don Frederico’s daughter, Ursula, and he also introduces his new friends to Enlightenment ideas. El Conde is especially taken with those ideas, which causes Father Sulpicio to announce that he’s opening another arm of the Inquisition. Not long after this, Don Frederico invites Cosimo to return to Granada with them and marry Ursula, but when the Spaniards receive the word to return home, Cosimo instead returns to Ombrosa. Back home, Cosimo woos many women. Biagio struggles to relate what exactly happened during this time, since he was touring Europe and only returned home when the Generalessa’s health declined. Not long after the Generalessa’s death, Ottimo Massimo takes off across a meadow owned by Duke Tolemaico that Cosimo cannot cross. A few days later, a beautiful woman rides into the meadow on a horse. It’s Viola, Duke Tolemaico’s widow. She agrees to meet Cosimo, seeming both angry and gentle, and their adult romance begins.
According to Biagio, this is the most wonderful part of Cosimo’s life. Cosimo and Viola have different ways of loving—Viola wants her lovers to tie themselves in knots and prove themselves, while Cosimo wants to think about love rationally—which often leads to fights. Viola often leaves for weeks after these fights, and once, while in Paris, a friend tells Biagio that Viola has other lovers all over Europe. Biagio passes this on to Cosimo, but he doesn’t believe it. Not long after, however, Viola begins to court the Englishman Sir Osbert and the Neapolitan Don Salvatore. She tries to convince the men to share her, which neither they nor Cosimo want to do. Eventually, Viola calls off her romances with all three men and leaves for England with Ottimo Massimo. Cosimo seems to go mad and declares that he’s a bird. He sets up a press in the trees and prints pamphlets railing against humanity. Things only begin to improve when wolves threaten Ombrosa, and Cosimo leads the effort to thwart them.
Biagio isn’t sure of the exact timeline, but sometime around this period, Cosimo becomes a Freemason. It suits him perfectly, given his connections with other Enlightenment thinkers in Europe and his home in the trees, but he eventually abandons the group—his life in the trees means that he has no interest in building things with bricks. By the time the local Masonic lodge becomes a proper lodge Cosimo isn’t involved. He does, however, meet up again with Father Sulpicio and two cronies, who are trying to infiltrate the Freemasons. Cosimo stabs Father Sulpicio in the stomach.
As Cosimo gets older, he begins to help with the grape harvest as he becomes light enough to walk across the trellises. When the peasants begin resenting how much of the grape harvest they send to nobles, Cosimo introduces them to the French Revolution idea of creating a “complaint book,” but this exercise does little but incite rebellion. One day during harvest, the peasants and Cosimo drive out the collectors and celebrate. Soon, however, the Austro-Sardinian army, led by the young Count d’Estomac, arrives to put a stop to the rebellion. Cosimo evades capture and spends the years of the French Revolution helping French soldiers, especially those led by Lieutenant Papillon. Briefly after the end of the war, Cosimo sits on the local council, but nobody reads or implements his plan for constitutional government. He feels duty-bound to support the French, but helps local peasants evade Napoleon’s cruel tactics. Napoleon does visit Cosimo, but not long after, Cosimo meets Russian soldiers chasing Napoleon out of their country. Cosimo falls ill but refuses to come out of the trees, so when an out-of-control hot air balloon passes by, he grabs onto it and drops himself into the sea to his death.
Biagio isn’t sure what comes next. The 19th century is going poorly, and the ideals of the Enlightenment are dead. He tries to study the newspapers, but without Cosimo, it’s meaningless. Further, all the trees Cosimo lived in are gone and now, the only trees are exotic ones. He wonders if Ombrosa only existed because Cosimo lived there.