The plot of the series focuses on Marco, a young boy who lives with his family in the harbor city of Genoa, Italy during a depression period in 1881. Marco's father, Pietro Rossi, is a manager of a hospital who dedicates his time to treating poor patients, and therefore the family has financial difficulties. His beloved mother, Anna Rossi, goes to Argentina to work as a maid to earn money for Marco. When the letters from his mother stop coming after an indication that she is sick, Marco fears the worst for his mother's fate, and since his father is too busy working in his clinic and his older brother Tonio was sent off to train as a locomotive driver in Milano, he is the only one free enough to go search for her.
Marco takes with him his older brother's pet monkey, Amedeo and together they sneak aboard the Andrea Dora, a ship bound for Brazil. In Brazil Marco boards an immigrant ship and finally arrives in Buenos Aires, where he meets a puppeteer called Peppino and his family, whom he knew from Genoa. They accompany him to Bahía Blanca in order to try to locate his mother.
In Bahía Blanca, he discovers his uncle stole the letters which his mother had sent him. He returns to Buenos Aires, and sails off on a ship to Rosario; there he tries to figure out how to get on a train to Córdoba. Marco's Italian friends collect enough money and buy him a train ticket. Marco arrives in Córdoba, and successfully manages to locate the agricultural engineer Mister Mequinez. He tells Marco that his mother works for his brother in Tucumán, and gives him enough money for a train ticket. But Marco ends up giving the money to a doctor to save the life of a poor girl he meets. Marco sneaks on the train but he gets caught and tossed off the train in the middle of nowhere. A group of travelling gipsies rescue him and give him an old donkey.
After a few days, the donkey dies and Marco continues to walk to Tucumán by foot. He eventually arrives to his destination hungry and tired, and finds his mother. His mother is very sick and needs an operation, but she is too weak. As soon as she sees Marco, she regains her strength and manages to go through surgery successfully.
At the end of the series, Marco and his mother return to Genoa, where the family is reunited.