Heart of Darkness centers around Marlow, an introspective sailor, and his journey up the Congo River to meet Kurtz, reputed to be an idealistic man of great abilities. The native inhabitants of the region have been forced into the Company's service, and they suffer terribly from overwork and ill treatment at the hands of the Company's agents.
Marlow arrives at the Central Station. He finds that his steamship has been sunk and spends several months waiting for parts to repair it. His interest in Kurtz grows during this period. The manager and his favorite, the brickmaker, seem to fear Kurtz as a threat to their position. Marlow eventually gets the parts he needs to repair his ship, and he and the manager set out with a few agents (Marlow calls them pilgrims because of their strange habit of carrying long, wooden staves wherever they go) and a crew of cannibals on a long, difficult voyage up the river.
Not long after, Marlow and his companions arrive at Kurtz's Inner Station, expecting to find him dead, but a half-crazed Russian trader assures them that everything is fine and that he is the one who left the wood. The Russian claims that Kurtz has enlarged his mind and cannot be subjected to the same moral judgments as normal people. Kurtz has established himself as a god with the natives and has gone on brutal raids in the surrounding territory in search of ivory.
The manager brings Kurtz, who is quite ill, aboard the steamer. A beautiful native woman, apparently Kurtz's mistress, appears on the shore. Kurtz disappears in the night, and Marlow goes out in search of him, finding him crawling on all fours toward the native camp. Marlow stops him and convinces him to return to the ship. Marlow listens to Kurtz talk while he pilots the ship, and Kurtz entrusts Marlow with a packet of personal documents, including an eloquent pamphlet on civilizing the savages which ends with a scrawled message that says, "Exterminate all the brutes!" The steamer breaks down, and they have to stop for repairs. Kurtz dies, uttering his last words—"The horror! The horror!"—in the presence of the confused Marlow. Marlow falls ill soon after and barely survives. Eventually he returns to Europe and goes to see Kurtz's Intended (his fiancée). She asks what his last words were, but Marlow cannot bring himself to tell her the truth. Instead, he tells her that Kurtz's last word was her name