A seasoned mariner narrates his voyage up a central African river to locate an influential ivory trader whose isolation and unchecked authority reveal the moral rot of European colonial enterprise. The layered frame narrative begins on the Thames and shifts into dense jungle, contrasting metropolitan civility with frontier brutality, and tracking the narrator's growing disquiet as civilization's pretenses crumble. Encounters with exploitative stations and spectral landscapes probe themes of power, madness, and the ambiguous boundary between enlightenment and darkness in the human heart.