An unnamed narrator describes Marlow and four companions on the Thames at dusk. Marlow begins his Congo story: his appointment through his aunt, the Brussels Company offices with their two knitting women, and his first glimpse of the 'heart of darkness' — the blank space on the map that he has wanted to explore since childhood.

Marlow arrives in Africa and begins his trek to the Central Station. At the Outer Station, he witnesses African workers being worked to death in a grove of trees. He meets a fastidiously dressed Company accountant who praises Kurtz for the first time, dismissing the dying Africans as an 'unfortunate' noise.

Marlow reaches the Central Station to find his steamboat wrecked. The Manager — a hollow man sustained only by not getting sick — reveals his distrust of Kurtz. Marlow waits months for rivets to repair the boat, overhearing the Manager scheming and listening to the jungle press in.

The steamboat travels up the Congo River into increasingly dense jungle. Marlow describes the sense of traveling backward in time, away from civilization. Fog closes in. African voices erupt from the banks. Marlow reflects on the thin restraint separating civilization from savagery — and finds it uncomfortably thin.

Marlow reaches the Inner Station and meets the Russian harlequin devotee of Kurtz. Kurtz is brought out on a stretcher — emaciated but enormous in voice and presence. His African mistress runs along the shore; his methods are revealed: skulls on fence posts, systematic raiding of villages. Kurtz attempts to return to his followers during the night; Marlow confronts him and brings him back.

Kurtz deteriorates on the return journey. He gives Marlow a packet of letters and papers for safekeeping. In his final hours, Kurtz peers into the darkness and pronounces his verdict: 'The horror! The horror!' He dies. Marlow falls seriously ill. Back in Brussels, he is left with Kurtz's shadow — and the need to speak to the Intended.

Marlow visits Kurtz's Intended in Brussels. She is still in mourning, sustained by the certainty that Kurtz was great. She asks for his last words. Marlow tells her 'your name.' The lie is told. The novella returns to the Thames, where darkness flows in from the sea.