The Baron in the Trees is a novel that follows Cosimo di Rondo, who is a baron in 18th-century Italy. The story itself is told through the eyes of Biagio, who is Cosimo’s little brother. Cosimo is sick of his life and especially sick of how his sister, Battista, treats him. Battista does cruel things to Cosimo, such as forcing him to eat snails.

At this point, Cosimo decides to climb a tree, saying that he’s so sick of his life that he will never return to the ground. The story follows Cosimo’s adventures living up in the trees, including keeping a library there. (In particular, Cosimo's library books are philosophical in nature, dealing with issues such as class inequality.) Later on, he becomes so fond of trees that he helps people in the local town put out a fire before it destroys the forest.

Other adventures Cosimo goes on include fighting pirates and meeting women to romance. Eventually, he reconnects with a childhood friend named Viola and falls in love with her. She does not reciprocate the love, however, and Cosimo retreats back into the trees. He still helps his family from time to time, but eventually, he grows old and jumps onto a hot air balloon, diving from there into a lake so that he never has to be on the ground ever again.