A group of men are aboard an English ship that is sitting on the Thames. The group includes a Lawyer, an Accountant, a Company Director/Captain, and a man without a specific profession who is named Marlow. The narrator appears to be another unnamed guest on the ship. While they are loitering about, waiting for the wind to pick up so that they might resume their voyage, Marlow begins to speak about London and Europe as some of the darkest places on earth.
The narrator and other guests do not seem to regard him with much respect. Marlow is a stationary man, very unusual for a seaman. The others do not understand him because he does not fit into a neat category in the same manner that the others do. He mentions colonization and says that carving the earth into prizes or pieces is not something to examine too closely because it is an atrocity. He then begins to narrate a personal experience in Africa, which led him to become a freshwater sailor and gave him a terrible glimpse of colonization. With the exception of two or three small paragraphs, the perspective shifts to Marlow, who becomes the main narrator for the rest of the novel.
Marlow has always had a passion for travel and exploration. Maps are an obsession of his. Marlow decides he wants nothing more than to be the skipper of a steamship that travels up and down a river in Africa. His aunt has a connection in the Administration Department of a seafaring and exploration company that gathers ivory, and she manages to get Marlow an appointment. He replaces a captain who was killed in a skirmish with the natives. When Marlow arrives at the company office, the atmosphere is extremely dim and foreboding. He feels as if everyone is looking at him pityingly. The doctor who performs his physical asks if there is a history of insanity in Marlow's family. He tells Marlow that nothing could persuade him to join the Company down in the Congo. This puzzles Marlow, but he does not think much of it. The next day he embarks on a one-month journey to the primary Company station. The African shores that he observes look anything but welcoming. They are dark and rather desolate, in spite of the flurry of human activity around them. When he arrives, Marlow learns that a company member recently committed suicide. There are multitudes of chain-gang types, who all look at him with vacant expressions. A young boy approaches Marlow, looking very empty. Marlow can do nothing but offer him some ship biscuits. He is very relieved to leave the boy behind as he comes across a very well-dressed man who is the picture of respectability and elegance. They introduce themselves: he is the Chief Accountant of the Company. Marlow befriends this man and frequently spends time in his hut while the Accountant goes over the accounts. After ten days of observing the Chief Accountant's ill temper, Marlow departs for his 200-mile journey into the interior of the Congo, where he will work for a station run by a man named Kurtz.
The journey is arduous. Marlow crosses many paths, sees deserted dwellings, and encounters black men working. Marlow never describes them as humans. Throughout the novel, the white characters refer to them in animalistic terms. Marlow finally arrives at a secondary station, where he meets the Manager, who for now will oversee his work. It is a strange meeting. The Manager smiles in a manner that is very discomfiting. The ship on which Marlow is supposed to set sail is broken. While they await the delivery of the rivets needed to fix it, Marlow spends his time on more mundane tasks. He frequently hears the name "Kurtz" around the station. Clearly everyone knows his future boss. It is rumored that he is ill. Soon the entire crew will depart for a trip to Kurtz's station.
The Manager's uncle arrives with his own expedition. Marlow overhears them saying that they would like to see Kurtz and his assistant hanged so that their station could be eliminated as ivory competition. After a day of exploring, the expedition has lost all of their animals. Marlow sets out for Kurtz's station with the Pilgrims, the cannibal crew, and the Manager. About eight miles from their destination, they stop for the night. There is talk of an approaching attack. Rumor has it that Kurtz may have been killed in a previous one. Some of the pilgrims go ashore to investigate. The whirring sound of arrows is heard; an attack is underway. The Pilgrims shoot back from the ship with rifles. The helmsman of the ship is killed, as is a native ashore. Marlow supposes that Kurtz has perished in the inexplicable attack. This upsets him greatly. Over the course of his travels, he has greatly looked forward to meeting this man. Marlow shares Kurtz's background: an English education, a woman at home waiting for him. In spite of Marlow's disappointment, the ship presses onward. A little way down the river, the crew spot Kurtz's station, which they had supposed was lost. They meet a Russian man who resembles a harlequin. He says that Kurtz is alive but somewhat ill. The natives do not want Kurtz to leave because he has expanded their minds. Kurtz does not want to leave because he has essentially become part of the tribe.
After talking for a while with the Russian, Marlow has a very clear picture of the man who has become his obsession. Finally, he has the chance to talk to Kurtz, who is ill and on his deathbed. The natives surround his hut until he tells them to leave. While on watch, Marlow dozes off and realizes that Kurtz is gone. He chases him and finds Kurtz in the forest. He does not want to leave the station because his plans have not been fully realized. Marlow manages to take him back to his bed. Kurtz entrusts Marlow with all of his old files and papers. Among these is a photograph of his sweetheart. The Russian escapes before the Manager and others can imprison him. The steamboat departs the next day. Kurtz dies onboard a few days later, Marlow having attended him until the end.
Marlow returns to England, but the memory of his friend haunts him. He manages to find the woman from the picture, and he pays her a visit. She talks at length about his wonderful personal qualities and about how guilty she feels that she was not with him at the last. Marlow lies and says that her name was the last word spoken by Kurtz—the truth would be too dark to tell her.