Marlow reaches the Inner Station and notices Kurtz's building through his telescope — there is no fence, but a series of posts ornamented with "balls" that Marlow later learns were natives' heads. A Russian trader and disciple of Kurtz, called "The Harlequin" by Marlow, approaches the steamboat and tells Marlow that Kurtz is still alive. Marlow learns that the hut they previously saw is the Harlequin's. The Harlequin speaks enthusiastically of Kurtz's wisdom, saying, "This man has enlarged my mind."
Marlow learns from him that the steamboat was attacked because the natives did not want Kurtz to be taken away. Suddenly, Marlow sees a group of native men coming toward him, carrying Kurtz on a stretcher; Kurtz is taken inside a hut, where Marlow approaches him and gives him some letters. Marlow notices that Kurtz is frail, sick, and bald. After leaving the hut, Marlow sees a "wild and gorgeous" native woman approach the steamer; the Harlequin hints to Marlow that the woman is Kurtz's Mistress. Marlow then hears Kurtz chiding the Manager from behind a curtain: "Save me! — save the ivory, you mean." The Harlequin, fearing what might happen when Kurtz is taken on board the steamboat, asks Marlow for some tobacco and rifle cartridges; he then leaves in a canoe.
At midnight that same night, Marlow awakens to the sound of a big drum. He inspects Kurtz's cabin, only to discover that he is not there. Marlow runs outside and finds a trail running through the grass — and realizes that Kurtz is escaping by crawling away on all fours. When he comes upon Kurtz, Kurtz warns him to run, but Marlow helped Kurtz to his feet and carried him back to the cabin.
The next day, Marlow, his crew, and Kurtz leave the Inner Station. As they move farther away from the Inner Station, Kurtz's health deteriorates; at one point, the steamboat breaks down and Kurtz gives Marlow a packet of letters and a photograph for safe-keeping, fearing that the Manager will take them. Marlow complies.
One night after the breakdown, Marlow approaches Kurtz, who is lying in the pilothouse on his stretcher "waiting for death." After trying to reassure Kurtz that he is not going to die, Marlow hears Kurtz whisper his final words: "The horror! The horror!" The next day, Kurtz is buried offshore in a muddy hole.
After returning to Europe, Marlow again visits Brussels and finds himself unable to relate to the sheltered Europeans around him. A Company official approaches Marlow and asks for the packet of papers to which Kurtz had entrusted him. Marlow refuses, but he does give the official a copy of Kurtz's report to The Society for the Suppression of Savage Customs with Kurtz's chilling postscript ("Exterminate all the brutes!") torn off. He learns that Kurtz's mother had died after being nursed by Kurtz's "Intended," or fiancée.
Marlow's final duty to Kurtz is to visit his Intended and deliver Kurtz's letters (and her portrait) to her. When he meets her, at her house, she is dressed in mourning and still greatly upset by Kurtz's death. Marlow lets slip that he was with Kurtz when he died, and the Intended asks him to repeat Kurtz's last words Marlow lies to her and says, "The last word he pronounced was — your name." The Intended states that she "knew" Kurtz would have said such a thing, and Marlow leaves, disgusted by his lie yet unable to prevent himself from telling it.
The anonymous narrator on board the Nellie then resumes his narrative. The Director of Companies makes an innocuous remark about the tide, and the narrator looks out at the overcast sky and the Thames — which seems to him to lead "into the heart of an immense darkness."