The tale is told at second hand. Aboard the cruising yawl Nellie, anchored in the Thames as the tide turns, an unnamed listener relays the story of Marlow, who sits cross-legged like a Buddha and begins to talk of the time he went up a great river in Africa. Before he starts, he reminds his companions that this dark place, too, was once one of the dark places of the earth, and that conquest, looked at closely, is mostly robbery and aggravated murder redeemed only by an idea.
Marlow takes a post with a Continental trading company and travels to its stations. From the first he is met by absurdity and cruelty: a warship shelling the bush, machinery rusting in the grass, and a shaded grove where worked-out African laborers have crawled away to die. Amid this he keeps hearing of one man, Kurtz, a first-class agent who sends down more ivory than all the others together and is spoken of as a remarkable, almost prophetic figure.
He patches up a wrecked steamer and pushes upriver toward Kurtz's Inner Station. The journey feels like travel back into the earliest beginnings of the world, the boat creeping past walls of vegetation while unseen drums beat and the crew glimpse villages erupting in yells and stamping. Marlow insists the unsettling part is not that these people are alien but that they are not: their humanity is kin to his own. Near the station the steamer is attacked, and his African helmsman is killed beside him.
Kurtz, when found, is desperately ill and half-mad, worshipped by the surrounding people, his outpost ringed with severed heads on stakes. His benevolent report on suppressing savage customs ends in the scrawled cry to exterminate the brutes. He has filled the empty places in himself with the wilderness and his own appetites. Carried down on the steamer and dying, he relives his life in a moment of complete knowledge and pronounces his final verdict, 'The horror! The horror!', before the manager's boy announces with contempt that he is dead.
Back in Europe Marlow, still shaken, visits Kurtz's Intended, a woman sustained by an idealized memory of the man. She begs to hear his last words to live with. Marlow, who has peeped over the edge himself and believes Kurtz's cry to be a glimpsed truth, finds he cannot give it to her; he says the last word Kurtz spoke was her name. The lie does not bring the heavens down. Marlow falls silent on the Nellie, and the listener notes that the river before them seems to lead into the heart of an immense darkness.