The narrator goes back in time to 1949, when Nirmal and Nilima met. Born in Dhaka in what became East Bengal, Nirmal chose to stay in Calcutta and India after partition. He was known as a leftist intellectual and a talented writer, and he taught Nilima's college English class. Nilima's family was wealthy and involved in public service. One afternoon, she followed Nirmal onto a bus. They married in less than a year, and Nilima's family refused to attend. Months later, the police came for Nirmal. They objected to his involvement in a Socialist International conference and detained him for a few days.
Nirmal and Nilima's early history situates both of them as intellectuals, while the fact that they met in an English class shows that they're both lovers of language. The fact that it leads to their marriage shows that language and literature have the power to bring people together and transform relationships. Nirmal is also a Communist at a time when the Indian government wasn't thrilled about the rise of alternative government forms, hence detaining him.
This had a profoundly negative effect on Nirmal. After Nilima begged her parents' forgiveness and had their doctors out to see Nirmal, they suggested he move out of the city. Nilima's father knew the island of Lusibari was looking for a schoolteacher. Though Nirmal was aghast at the thought of working at the capitalist Hamilton Estate, he and Nilima went for a visit and were astonished by what they saw—major reverence for Sir Daniel, who addressed local poverty far more effectively than even radical Nirmal had imagined possible. They moved months later.
When both Nirmal and Nilima are sobered by seeing that someone like Sir Daniel was actually able to put their radical ideas into practice, it reinforces for both of them the importance of taking action over dwelling on theory and just talking about action, which the novel implies is mostly what they did during their time in Calcutta.
The first few months were hard. Hunger and catastrophe reigned in tide country; the ground was still salty and bore poor crops, which sent people instead to hunt and fish. Doing that, they drowned and were easy prey for tigers, sharks, and crocodiles. The school was grossly underfunded; all the money apparently went to the corrupt landowners. Nirmal was entirely overwhelmed and read Lenin's pamphlet. Nilima began to speak to the local women. She was shocked to learn that women in tide country expect to be widows in their twenties, and wives dressed as widows when their husbands went out on their boats. This was unthinkable to Nilima.
What Nilima and Nirmal find is that residents in tide country live in deference to nature, while residents in Calcutta live in a city where nature is, by default, second to manmade structures and systems. The fact that Nirmal is so overwhelmed suggests that he'll struggle with taking action in the future, while Nilima's choices suggest that she took Sir Daniel's actions to heart and will try to do something about what she sees.
Nilima struggled to name this group of women, and Nirmal objected to calling the widows a class—it implied a division incompatible with Communism. Finally, she decided it was more important to help them than name them, which led to the Women's Union and later, to the Babadon Trust. In a few years, the landowners were pushed out, and the Trust provided many services. Nirmal was dismissive of Nilima's efforts, as they were anti-Communist social services, but he did give the Babadon Trust its name.
When Nilima decides that helping people is more important than naming them, it's a clear indicator that she values taking action over thinking about theory. Nirmal's opposition to this shift suggests that he cares deeply for his theory, and that it will be impossible for him to ever put it into practice.