The story starts on the board of British ship “Nellie”, anchored on the river Thames. A sailor named Mallow tells his fellows about his past travels. One of these was his journey to South Africa. Mallow was hired by “The Company” to work as its agent and to be involved in ivory trade. The task sounded easy enough, even despite Marlow predecessor on this position was murdered.
Marlow agreed and soon started his work as the riverboat captain going up to the river Congo. His task was just to reach the Company’s ship… or so he thought. On Central Station where Mallow rested and replenished his supplies, he met its general manager - a strange man wearing full-formal outfit despite the hellish heat outside. That man told Marlow about his current trouble - the broken steamship - and about a missing man named Kurtz, who was rumored to be both prisoner and demigod for the local tribe. Kurtz was an agent from another branch of the Company but he suddenly dismissed his assistant and, according to his words, became seriously ill and could be in need of emergency help.
Marlow spent several months to get the details needed and help in ship repair. During that time, he observed the sharp contrast between life of the white people and natives. Ivory traders cruelly exploited the tribesmen, working them to death and exploiting them in every way possible. On the other hand, there was the wild jungle, so mysterious and terrifying that the settlements looked like fragile islands in the unknown and merciless ocean. The habits of the local tribes were strange and incomprehensible, some of them, such as cannibalism, disgusted and frightened Marlow, but the ivory traders used it as a reason to tread locals as they did, as a proof they weren’t fully humans and didn’t deserve anything else.
During the ship repair, Marlow became more and more intrigued by Kurtz’s personality. Who was he and why the general manager and other personnel were so afraid of him? Was it some power play amongst the agents of the company or was there really something so wrong with Kurtz? Marlow gathered a crew of some other agents and local cannibal tribesmen and sailed to search for Kurtz. He called the agents “pilgrims” because of the long wooden staffs they carried with them everywhere. Such a crew was a source of anxiety itself (thankfully, the cannibals had hippo meet and weren’t hungry), but the things got worse from the very start of the journey.
Their first stop was to gather wood, but what was a surprise when they found a stack of already prepared wood in an abandoned hut. On top of the woodpile lied a note warning them to proceed cautiously. This note was exceptionally relevant later, when the boat had to stop because of dense fog and then attacked by the native tribe, unseen between the trees. One native member of the crew has been killed with their spear before Marlow got a bright idea to frighten the attackers away with the boat whistle.
When they finally reached Kurtz’s camp, it was surrounded by severed heads impaled on the sticks. They expected to find him dead, but the heads belonged to locals only. On the station there was no one except one Russian man who was dressed weirdly and didn’t look mentally healthy. The Russian told the crew that everything there was in perfect order and Kurtz just became a god, so there was no reason to worry about him. Kurtz grew beyond the petty human morals, so he had the right to impale locals and execute them with extreme prejudice for some barbarous rituals. The Russian cheerfully added that it was he who prepared the wood and the note for them to help them reach the camp safely.
Not satisfied with this answers, Marlow searched the station to find Kurtz himself and succeed. Kurtz was so ill that he couldn’t walk by himself anymore. The crew used a stretcher to take him to the ship, but the locals in the camp tried to kill them and defend their god.However, Kurtz wasn’t completely out of his mind. He told some words to the natives in their language, and they stepped back.
Despite Kurtz’s attempts to run - or rather crawl - away, back to his godlike status, the crew managed to take him to the ship safely. The Russian came with them, pointing to the beautiful native woman standing on the beach and looking at the boat. He said she was Kurtz’s lover and, possibly, lots of what he did were her ideas. He also told Marlow that it was Kurtz who sabotaged the ship to prevent general manager from spoiling his plans to collect ivory being a “god”. Then the Russian took a canoe and sailed away, refusing to go with the rest because he was afraid of the general manager’s anger.
While they travelled back to the mouth of Congo, Kurtz’s disease progressed. He almost lost his sight, was unable to move completely and could barely speak. Kurtz had fever and spoke strange and random things, but in the rare moment of sanity he gave a pack of personal documents to Marlow and asked him to hide those documents from the Company and tell his fiancée about his death. The fever episodes became more and more exhausting and terrifying and soon afterwards Kurtz died painfully. His last words were “The horror! The horror!”
The disease seemed to be contagious because Marlow, who spent time with Kurtz, became sick also. He survived through the same fever and horror, but, unlike Kurtz, he managed to recover back to health and sanity. Marlow read the documents left to him by Kurtz. That was his log of falling into madness of some kind. The first pages of the stack were an eloquent description of the plan of educating and civilizing the locals but with every new page Kurtz became more disillusioned and, seemingly, ill. The handwriting changed to simpler, the phrases became shorter and the last note scrawled on the page said “Exterminate all the brutes!”.
After returning to Europe, Marlow felt embittered and disgusted with the policy of the Company and the “civilized society” that accepted that as normal. He really refused to give Kurtz’s documents to the authorities of the Company, keeping his word but possibly destroying his career. Marlow found a Belgian girl, Kurtz’s fiancée, who was still in mourning after him, despite more than a year passing from his death. She told Marlow about what a wonderful person Kurtz was and tearfully asked him about his last words. Marlow couldn’t tell her the truth about Kurtz and break her heart once more. So he said that the last thing Kurtz whispered in his life was her name.