Section 3 of Heart of Darkness begins with Marlow's astonishment at how the Russian got this far into the jungle. Marlow learns more about Kurtz from the Russian, who is enthralled by Kurtz's charisma. Marlow observes that the Russian does not appear to be of sound mind. Marlow learns how Kurtz obtains his ivory through raids on nearby tribes. Taking in this information, Marlow presumes that Kurtz has become mentally unstable.

Eventually, Marlow sees the sickly Kurtz come by, carried on a stretcher. Marlow then sees a group of armed locals standing across the river. The manager appears and tells Marlow and the Russian that Kurtz has damaged the company through his unsound practices. Before leaving, the Russian extracts a promise from Marlow to protect Kurtz's reputation.

That night, Marlow finds Kurtz crawling across the ground trying to reach a group of locals who are beating drums and chanting on a hill. Marlow helps Kurtz return to the station, concluding that his soul has gone mad but his mind is still his own.

The next day, as a group of Africans gathers on the shore, Marlow takes the boat away from the station with Kurtz on board. The Africans seem very upset at this development, which has the company's party nervous. To prevent his colleagues from shooting at the locals, Marlow sounds the boat's whistle and the people on shore scatter. Soon afterward, the boat breaks down, and a nervous Kurtz entrusts a packet of letters and a photograph to Marlow. Later that day, Kurtz dies. The following day, the Marlow and the crew bury Kurtz in a muddy hole.

Marlow returns to Europe and feels a newfound resentment for the people there. Eventually, after some prodding by the company, Marlow gives his official report. Marlow meets with a few people who are interested in what happened to Kurtz, such as Kurtz's cousin and a journalist, both of whom hold Kurtz's memory in high regard.

The following year, Marlow goes to Kurtz's fiancée to give her the letters and the photograph that Kurtz gave him. Unable to tell the mourning woman Kurtz's actual final words ("The horror! The horror!"), he lies and says that in his final breath he uttered her name.

The story ends where it began, back aboard the Nellie in London. Marlow has finished his account, and the captain says that the tide is now favorable and they are ready to get underway.