Heart of Darkness (1899) is a novella by Polish-English novelist Joseph Conrad, about a voyage up the Congo River into the Congo Free State, in the heart of Africa, by the story's narrator Charles Marlow. Marlow tells his story to friends aboard a boat anchored on the River Thames, London, England. This setting provides the frame for Marlow's story of his obsession with the ivory trader Kurtz, which enables Conrad to create a parallel between London and Africa as places of darkness.

Where does the heart of darkness take place? Let's have some stream of consciousness first.
To answer this question, we should know that: Heart of Darkness (1899) is a novella by Polish-English novelist Joseph Conrad, about a voyage up the Congo River into the Congo Free State, in the heart of Africa, by the story's narrator Charles Marlow. So, the answer is the Congo River into the Congo Free State.