Marco Rossi, a young boy residing in Genoa, Italy, endures the hardships of a late 19th-century economic depression. Financial strain compels his mother, Anna Rossi, to seek employment in Argentina. After initial letters, her correspondence ceases, fueling Marco's distress over her safety. Resolute in finding her, Marco undertakes an arduous journey from Italy to Argentina, accompanied by Amedeo, a pet monkey originally belonging to his older brother Tonio. He stows away on ships, including the Andrea Doria bound for Brazil, before reaching Buenos Aires.

In Argentina, Marco encounters Genoese acquaintances—the puppeteer Peppino and his family—who aid his travel to Bahía Blanca. There, he uncovers his uncle’s interception and withholding of his mother’s letters. Undeterred, Marco presses on to Rosario and Córdoba. An agricultural engineer, Mister Mequinez, reveals Anna’s location in Tucumán and provides train fare. Marco selflessly diverts the funds to save a critically ill girl, compelling him to sneak aboard the train ticketless. Discovered and ejected in a desolate region, he is rescued by Roma travelers who gift him a donkey. After the donkey perishes, Marco traverses harsh terrain on foot, battling exhaustion and starvation.

He arrives in Tucumán to find his mother severely ill and requiring surgery. Her spirits lift instantly upon seeing Marco, granting her strength for a successful operation. Recovered, they return to Genoa, reuniting their family. The 1999 animated film remake adheres strictly to this narrative trajectory, depicting Marco’s journey, sacrifices, and reunion without deviation. No additional official media explores Marco’s background or development beyond this core story.