Charles Marlow tells his friends the story of how he became captain of a river steamboat for an ivory trading company. As a child, Marlow was fascinated by "the blank spaces" on maps, particularly Africa. The image of a river on the map particularly fascinated Marlow.
In a flashback, Marlow makes his way to Africa, taking passage on a steamer. He travels 30 mi (50 km) up the river where his company's station is. Work on a railway is taking place. Marlow explores a narrow ravine, and is horrified to find himself in a place full of critically ill Africans who worked on the railroad and are now dying. Marlow must wait for ten days in the company's devastated Outer Station. Marlow meets the company's chief accountant, who tells him of a Mr. Kurtz, who is in charge of a very important trading post, and is described as a respected first-class agent. The accountant predicts that Kurtz will go far.
Belgian river station on the Congo River, 1889Marlow departs with 60 men to travel to the Central Station, where the steamboat that he will command is based. At the station, he learns that his steamboat has been wrecked in an accident. The general manager informs Marlow that he could not wait for Marlow to arrive, and tells him of a rumour that Kurtz is ill. Marlow fishes his boat out of the river and spends months repairing it. Delayed by the lack of tools and replacement parts, Marlow is frustrated by the time it takes to perform the repairs. He learns that Kurtz is resented, not admired, by the manager. Once underway, the journey to Kurtz's station takes two months.
The Roi des Belges ("King of the Belgians"—French), the Belgian riverboat Conrad commanded on the upper Congo, 1889The journey pauses for the night about 8 miles (13 km) below the Inner Station. In the morning the boat is enveloped by a thick fog. The steamboat is later attacked by a barrage of arrows, and the helmsman is killed. Marlow sounds the steam whistle repeatedly, frightening the attackers away.
After landing at Kurtz's station, a man boards the steamboat: a Russian wanderer who strayed into Kurtz's camp. Marlow learns that the natives worship Kurtz and that he has been very ill. The Russian tells of how Kurtz opened his mind and admires Kurtz even for his power and his willingness to use it. Marlow suspects that Kurtz has gone mad.
Marlow observes the station and sees a row of posts topped with the severed heads of natives. Around the corner of the house, Kurtz appears with supporters who carry him as a ghost-like figure on a stretcher. The area fills with natives ready for battle, but Kurtz shouts something and they retreat. His entourage carries Kurtz to the steamer and lays him in a cabin. The manager tells Marlow that Kurtz has harmed the company's business in the region because his methods are "unsound". The Russian reveals that Kurtz believes the company wants to kill him, and Marlow confirms that hangings were discussed.
Arthur Hodister (1847–1892), who Conrad's biographer Norman Sherry has argued served as one of the sources of inspiration for KurtzAfter midnight, Kurtz returns to shore. Marlow finds Kurtz crawling back to the station house. Marlow threatens to harm Kurtz if he raises an alarm, but Kurtz only laments that he did not accomplish more. The next day they prepare to journey back down the river.
Kurtz's health worsens during the trip. The steamboat breaks down, and while stopped for repairs, Kurtz gives Marlow a packet of papers, including his commissioned report and a photograph, telling him to keep them from the manager. When Marlow next speaks with him, Kurtz is near death; Marlow hears him weakly whisper, "The horror! The horror!" A short while later, the manager's boy announces to the crew that Kurtz has died (the famous line "Mistah Kurtz—he dead" would become the epigraph of T. S. Eliot's poem The Hollow Men). The next day Marlow pays little attention to Kurtz's pilgrims as they bury "something" in a muddy hole.
Returning to Europe, Marlow is embittered and contemptuous of the "civilised" world. Several callers come to retrieve the papers Kurtz entrusted to him, but Marlow withholds them or offers papers he knows they have no interest in. He gives Kurtz's report to a journalist, for publication if he sees fit. Marlow is left with some personal letters and a photograph of Kurtz's fiancée. When Marlow visits her, she is deep in mourning although it has been more than a year since Kurtz's death. She presses Marlow for information, asking him to repeat Kurtz's final words. Marlow tells her that Kurtz's final word was her name.
