The question of the setting of Heart of Darkness by Joseph Conrad is complicated by the layering of the narration. In one sense, one could say that the story is entirely set in England, on a boat moored in the Thames river near London, where Charlie Marlow tells his story to a group of four other men.
Marlow talks about his travels around the world as a sailor, but his story proper begins in Brussels, where he meets with an employer and is hired as a steamer captain to pilot one of the company's ships on the Congo River, in what was then known as the Belgian Congo. The main setting of the story is Africa, where Marlow tells of traveling along the river to successively more remote trading stations to find Kurtz, the manager of the inner station. The trajectory of the journey moves from areas completely controlled by white Europeans to areas in which a limited number of white colonists and traders live, into the "heart of darkness," parts of Africa barely touched by the colonial powers.
The story is based on Conrad's own voyages to the Congo when he was employed in the French merchant navy, and descriptions of Europeans in colonial Africa. Their fraught relationships with Africans, as well as the descriptions of the scenery and the ships, are quite realistic, albeit from the point of view of white colonial employees rather than from the viewpoint of Africans.