1920. Harihar, Sarbajaya and their ten-year-old son Apu, live in the Temple City of Bananas (Varanasi) on the banks of the holy river Ganga (Ganges). Harihar earns a meager living by reciting religious scriptures. The film opens with Apu wandering and exploring the city. He also encounters their neighbor Nanda Babu, who would soon make a pass at Sarbajaya.
Harihar falls ill with fever and collapses at the riverbank. In the early hours of the next morning, Sarbajaya wakes Apu to fetch holy water from the river to put in his father's mouth as he is dying. Harihar's death leaves mother and son to fend for themselves.
The mother decides to return with Apu to live in a village where an old uncle works as a priest. Apu's mother works to support the family. Apu is initiated into priesthood and takes over the old man's work. He is unhappy because he wants to go to school. Apu persuades his mother to send him to school. She makes sacrifices so that he might pursue his studies.
Apu, now sixteen, wins a scholarship and departs for Calcutta, leaving her alone. It breaks Sarbajaya's heart, but she relents. Her health is failing, and the loneliness in the village takes its toll.
Engulfed in city life - studying during the day and working in a printing press at night to pay for his expenses - Apu grows away from his mother. His visits get shorter as the time passes. This emotional distance unnoticed by the growing Apu, hurts Sarbajaya deeply. She waits silently for her son's visit as her illness accelerates and falls into a depression.
On a night sparkling with dancing fireflies, Sarbajaya dies. Apu comes back to an empty house. He grieves for his mother, but soon finds strength to leaves the village for the last time, to carry on with his new life in the city...