Ishwar and his little sister Sita come to West Bengal from Bangladesh after the Partition. They land up in a refugee camp. One day, they witness a lower-caste woman being separated from her son, Abhiram. Ishwar decides to take care of the young boy.
Soon after this, Ishwar’s friend Rambilas helps him land a job in a factory in Chatimpur by the banks of the river Subarnarekha. Mr. Mukherjee, the foreman of the factory, ingratiates himself with Ishwar and becomes a part of their small family. Abhiram is sent to school in a nearby town while Sita grows up in Chatimpur. Ishwar plans to send Abhiram to Germany to study engineering. Abhiram, however, has other plans: he wants to be a writer. By this time, Sita and Abhiram have fallen in love. Ishwar is opposed to their union since Abhiram is born of a lower-caste woman. Around the same time, Abhiram meets his long-lost mother just as she is breathing her last in a railway station.
Ishwar asks Abhiram to leave for Calcutta and arranges for Sita to be married to someone else. Sita elopes with Abhiram on the night of her wedding, and they settle in Calcutta. The newlywed couple has a son, and Abhiram gets a job as a bus driver. Unfortunately, he accidentally runs over a young girl, and an angry mob lynches him.
A widowed Sita is forced to take up prostitution in order to make ends meet. Meanwhile, Ishwar leads a lonely life in Chatimpur. When his old friend Haraprasad comes to meet him, they come to Calcutta and end up getting drunk. Intoxicated, they head to a brothel and Ishwar lands in Sita’s room. Ashamed, Sita commits suicide. On the verge of losing his sanity, Ishwar realizes that his nephew Binu is now an orphan. He takes Binu to Chatimpur with him, only to find that he has been replaced by the foreman Mukherjee. He receives a letter from Mukherjee asking him to vacate his quarters. A newly fortified Ishwar leaves with Binu to seek a new future elsewhere.
Preceded by the acclaimed films Meghe Dhaka Tara (1960) and Komal Gandhar (1961), Subarnarekha is the third film in Ritwik Ghatak’s ‘Partition Trilogy’.