Marco Rossi is a ten-year-old Italian boy residing in financially struggling Genoa during Italy's 1881 economic depression. His father, Pietro Rossi, directs a charity hospital for the poor, deepening their hardship. Marco's mother, Anna Rossi, emigrates to Argentina for domestic work to support them. When her letters cease after mentioning illness, Marco resolves to find her.

Despite his youth, Marco exhibits maturity yet feels excluded by his father and older brother Tonio from significant family decisions, fueling his independent spirit. He secretly takes jobs, including at a wine shop, to fund his travel while continuing his education. Learning his mother's health might be failing intensifies his determination, leading him to embark on a transatlantic journey against his father's wishes.

Accompanied by Tonio's pet monkey Amedeo, Marco stows away on the ship *Andrea Doria* bound for Brazil, working as a cook's assistant to repay his passage. He transfers to an immigrant ship reaching Buenos Aires, where he is immediately robbed. There, he encounters the Peppino puppeteer family—Pepe and his daughters Concetta, Fiolina (Violetta), and Giulietta—previously known in Genoa. They shelter him and aid his wagon journey to Bahía Blanca, funded by puppet shows.

In Bahía Blanca, Marco discovers his uncle Francesco Merelli intercepted Anna's letters and stole her earnings. This betrayal forces Marco back to Buenos Aires, then onward to Rosario and Córdoba. Traveling across Argentina, Marco consistently demonstrates empathy and resilience. He surrenders his train fare to pay for medical treatment for a critically ill girl named Juana and endures abandonment after being caught stowing away on a train. He traverses the harsh Pampas and Andes, aided temporarily by Romani travelers who provide a donkey; after the donkey perishes, he continues on foot.

Marco's journey concludes in Tucumán, where he locates his gravely ill mother. Anna lacks the will for necessary surgery until Marco's arrival revitalizes her spirit, enabling a successful operation. Following her recovery, they return to Genoa for a family reunion. Departing Argentina, Marco reconnects with Fiolina, affirming their deep bond forged through shared hardship and expressing hope for a future reunion in Italy.

Marco's development showcases growing self-reliance, perseverance through physical and emotional trials, and unwavering commitment to family. His interactions across diverse cultural and socioeconomic landscapes highlight adaptability and innate kindness.