Charles Marlow, the narrator, tells his story to friends aboard
Nellie, a boat anchored on the
River Thames near
Gravesend, 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 departs 30 mi (50 km) up the river where his company's station is. Work on a railway is going on. Marlow explores a narrow ravine, and is horrified to find himself in a place full of diseased 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 a widely respected, first-class agent. The accountant predicts that Kurtz will go far.
Marlow departs with sixty men to travel to the Central Station, where the steamboat that he is to captain 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 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 of late. The Russian tells of how Kurtz opened his mind and seems to admire Kurtz even for his power and his willingness to use it. Marlow suggests 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, the manager appears with the pilgrims, bearing a gaunt and ghost-like Kurtz. The area fills with natives ready for battle, but Kurtz shouts something from the stretcher and the natives retreat. The pilgrims carry Kurtz to the steamer and lay him in one of the cabins. The manager tells Marlow that Kurtz has harmed the company's business in the region, that his methods are "unsound". The Russian reveals that Kurtz believes the company wants to kill him, and Marlow confirms that hangings were discussed.
After midnight, Marlow discovers that Kurtz has returned to shore. He finds Kurtz crawling back to the station house. Marlow threatens to harm Kurtz if he raises an alarm, but Kurtz only laments that he had not accomplished more. The next day they prepare to journey back down the river.
Kurtz's health worsens during the trip and Marlow becomes increasingly ill. 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 away 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 rest of the crew that Kurtz has died. The next day Marlow pays little attention to the pilgrims as they bury "something" in a muddy hole. He falls very ill, himself near death.
Upon his return 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.