Italo Calvino's "The Baron in the Trees" is the story of an Italian boy who leaves his aristocratic childhood home in favor of the expanse of adjoining trees that cover the surrounding town and countryside and lives the remainder of his life in the world he finds there. His name is Cosimo Piovasco di Rondo, and he applies his ingenious and free-thinking perspective to finding ways of continuing to learn, innovating both for the betterment of his own lifestyle and for the people who live below him, and to cultivating a one-of-a-kind, passion-filled love life. He lives at once removed from and intimately bonded with his family and fellow townspeople, and dies as creatively and note-worthily as he lived, leaving his friends and family inspired by his story.

Cosimo's story is told from the perspective of his younger brother, Biagio, and early partner in mischief and exploration. He recounts the events both as he remembers them, and as they were recounted to him by his brother, since he was not present to observe all of them. His admiration and that of the townspeople remain and spur the reader on to falling ever more deeply in love with the story's hero.

His early youth finds Cosimo befriending and becoming teacher and defender to a group of poor boys who climb into the trees to steal fruit, and a young girl who lives in the house with the exotic garden next door to his own. He makes an arrangement with the local book seller to keep himself in steady supply of books, and so becomes a student of the same philosophers and thinkers who fed the ideals of the French revolution. The same arrangement allows him to befriend and pass lighter reading along to a famous thief, the engrossment in which replaces his love for the life of crime.

As he grows, he becomes protector of the trees against fire, uniting the townspeople in the cause. He foils pirates and saves the reputation of his father's half-brother when he discovers he had been cooperating with them. Cosimo loves telling his stories, in widely varied versions, and thereby becomes a rather beloved character among the townspeople. In a particularly long adventure, Cosimo has his first taste of love among a group of Spanish exiles, and among the same group gets his first chance to solidify and defend the philosophical perspective he gleans from his books against a monk intent to bow to the prejudices of the Catholic Church.

As a young man, Cosimo returns to his hometown, and continues to apply his ingenuity, now to the goal of entertaining girls in his trees. Several are attracted to his singular lifestyle and lovable nature, and his legend grows even more as a result. It is not until the return of his childhood neighbor, however, that his own heart is captured and permanently fastened to one girl above all others. She remembers him and their childhood games with as much fondness as he does, and the two of them establish a highly hospitable household complete with bed, curtains and everything else needed for their regular rendezvous. Their love is strained beyond their willingness to admit their faults, however, when Cosimo's brother realizes that, while she is devoted entirely to Cosimo, the girl Violente also remains determined to encourage strings of other men to court her extravagantly. Thus ends his life's romance.

Cosimo's aging years find him ever more eccentric, caring with his siblings for their sick mother, still from his perch in the trees, and involving himself with secret societies such that their traditions take on a flavor distinctly his, while he remains free and unconstrained by any of them. When he must finally admit that he is too ill to live much longer, he leaps to catch a passing hot-air balloon and drops from his hold on it into the lake below, flamboyantly ending his passionate life without ever touching his feet to the ground.