Apu Roy (Soumitra Chatterjee) is an unemployed graduate living in a rented room in Calcutta. Despite his teacher's advice to go to university, he is unable to do so because he can't afford it. He tries to find a job, while barely getting by providing private tuition. His main passion is writing a novel, partially based on his own life, hoping to get it published some day. One day he meets his old friend Pulu, who coaxes him to join him on a trip to his village in Khulna to attend the marriage of a cousin named Aparna (Sharmila Tagore).
On the day of the marriage it turns out that the bridegroom has a serious mental disorder. The bride's mother cancels the marriage, despite the father's protests. He and the other villagers believe, according to prevalent Hindu tradition, that the young bride must be wedded off during the previously appointed auspicious hour, otherwise, she will have to remain unmarried all her life. Apu, after initially refusing when requested by a few villagers, ultimately decides to take Pulu's advice and come to the rescue of the bride by agreeing to marry her. He returns with Aparna to his apartment in Calcutta after the wedding. He takes up a clerical job, and a loving relationship begins to bloom between them. Yet the young couple's blissful days are cut short when Aparna dies while giving birth to their son, Kajal. Apu is overcome with grief and holds the child responsible for his wife's death.
He shuns his worldly responsibilities and becomes a recluse – travelling to different corners of India, while the child is left with his maternal grandparents. Meanwhile, Apu throws away his manuscript for the novel he had been writing over the years. A few years later, Pulu finds Kajal growing wild and uncared for. He then seeks out Apu, who is working at a mining quarry and advises Apu one last time to take up his fatherly responsibility. At last, Apu decides to come back to reality and reunite with his son. When he reaches his in-laws' place, Kajal, having seen him for the first time in his life, at first does not accept him as a father. Eventually he accepts Apu as a friend and they return to Calcutta together to start life afresh.