Aboard the Nellie, a small boat anchored on the Thames, a group of five male passengers gather in a meditative mood. The passengers include the Director of Companies, the Lawyer, the Accountant, Charles Marlow, and the narrator. As the sun sets, they talk. Marlow, the only man among them who still “followed the sea” (77), begins to talk about the darkness of Britain and how strange it must have seemed to the Roman troops who arrived millennia earlier. He ruminates on the dark tendencies of colonial powers before halting and beginning a story, telling how he travelled up the Congo River as the captain of a steamboat.

When Marlow was young, he had a passion for the blank spaces on maps. After returning from years working aboard ships in the Indian Ocean, unable to find new work, he uses his family connections to take charge of a steamboat on the Congo River. An ivory trading company has lost a captain in a dispute with the locals over chickens and needs a replacement. Within 48 hours, Marlow travels across the English Channel to his new employer’s offices. He examines a giant map marked in the colors of the imperial powers; Belgium’s yellow is seemingly small, but he spots the Congo River “dead in the center […] fascinating—deadly—like a snake” (82).