Nello Daas, a young orphan, lives in poverty with his elderly grandfather, Jehan Daas, in a modest hut near Antwerp, Belgium. After his mother's death, they survive through milk deliveries aided by their loyal dog, Patrasche, whom they rescued from abuse. Nello possesses a gentle, resilient, and artistic nature, maintaining kindness and optimism despite hardship. He teaches himself drawing, inspired by Peter Paul Rubens, and dreams of becoming an artist. His talent finds recognition from his friend Alois Cogez, though her wealthy father disapproves of their friendship due to Nello's poverty.

Key relationships shape his existence: his profound bond with Patrasche provides both work assistance and companionship. His grandfather nurtures him, encouraging his artistic aspirations. Anime adaptations introduce supportive friends like George and Paul; George becomes particularly loyal after Nello saves Paul from drowning.

Nello faces severe adversity, including false accusations of arson by a landlord named Hans. These accusations lead to his eviction following Jehan's death. Hoping to win prize money for rent, Nello enters a junior art contest but loses, deepening his despair. Homeless and destitute, he seeks solace in Antwerp Cathedral to view Rubens' paintings but cannot afford entry. On Christmas Eve, he and Patrasche enter the unlocked cathedral and freeze to death before the artwork.

In the 1975 anime adaptation, Nello's age is reduced from the novel's 15 to 10, heightening his vulnerability. This version softens the ending, depicting angels guiding their souls to heaven for a reunion with Jehan and Nello's mother. Other adaptations, such as the 1959 and 1999 American films, alter the ending to spare their lives.