Nirmal comes home sick from Morichjhãpi, struck by Kusum's insight. He spends days experiencing visions and reading Rilke. Nilima is angry with Horen, but gentler with Nirmal. She does ask if the rumors are true that he's involved with Kusum, which Nirmal denies. Nilima asks Nirmal to help Kusum by bringing her to Lusibari to work for the Trust, but Nirmal doesn't say that he doesn't think he can do anything of worth for the Trust. On Morichjhãpi, however, he knows he can do something.
Despite Nirmal's insistence that he couldn't be of use to the Trust, it's worth noting that in spite of his habit of getting bogged down in theory, he clearly has a lot to offer the Trust—if only he'd compromise and accept that the theories he loves so much won't be able to be implemented in full. This then shows Nirmal actively turning away from a life of action.
One afternoon when Nirmal is feeling better, he starts to go downstairs but stops when he hears Nilima talking with the doctor. They discuss Morichjhãpi and Nirmal's interest in the island, and decide it's best to sedate Nirmal and not tell him that there's going to be an attack on Morichjhãpi. Nirmal packs a bag and sneaks out to go find Horen. Horen has more information—the government has bused in criminals and gangsters to attack Morichjhãpi. They decide to try to make it to Morichjhãpi that night. Horen skillfully avoids police boats and steers his boat into the mud on Morichjhãpi.
Though Nirmal's narration mostly treats Horen as a means of transportation and a guide, it's worth considering why exactly Horen is so interested in going to Morichjhãpi in the first place. It could be that he's just as interested in the Marxist theory in play as Nirmal is, though it's also possible that he believes that his assistance is a way for him to make an impact on people even less fortunate than he is.
When Horen and Nirmal find Kusum, they find her happy and entirely unwilling to leave the island. She agrees to let Horen take Fokir away. Horen decides he'll leave the next night and is shocked when Nirmal announces that he's going to stay. He spends the next day filling this notebook and finally, runs out of time. Addressing Kanai directly, he suggests that maybe he'll know what to do with the notebook.
Kusum's unwillingness to leave suggests that she took some of Nirmal's love of theory to heart and was willing to die for the principle of what she was doing. The novel, however, overwhelmingly suggests that this kind of action is less valuable than other kinds, given that nobody even knows what happened or will talk about it.
Kanai's hands are shaking when he finishes. He finds Horen sitting outside and sits down with him. When Kanai mentions finishing the notebook, Horen matter-of-factly says that he should've left Morichjhãpi with Fokir earlier. He explains that what happened next is just rumors: the gangsters burned dwellings, sank boats, and did unimaginable things. Kusum was possibly taken away with a group of women, raped, and then thrown into the river. Nirmal was put on a bus to central India, but he was presumably let off in Canning. When Kanai asks why Nirmal was so interested in Morichjhãpi, Horen says that Nirmal was a bit mad. He brushes Kanai off when he asks if Nirmal was in love with Kusum.
Again, even though Nirmal's notebook recorded much of what happened in the lead-up to the massacre, it's clear that she was one of many faceless individuals who were killed to make a point about the power of the government. Remember too that the government clearly prioritized Morichjhãpi's use as a nature reserve over its use as a refuge for poor settlers, which shows again how the world's poor are often the ones to suffer the consequences of conservation efforts.