The novella opens and closes with imagery of a great river—not the Congo, where most of the plot is set, but rather the Thames in London. This is where the book’s frame story takes place, as Marlow tells his tale to a group of fellow sailors. The book’s second paragraph in Part 1 sets the scene:

The sea-reach of the Thames stretched before us like the beginning of an interminable waterway. In the offing the sea and the sky were welded together without a joint, and in the luminous space the tanned sails of the barges drifting up with the tide seemed to stand still in red clusters of canvas sharply peaked, with gleams of varnished spirits. A haze rested on the low shores that ran out to sea in vanishing flatness. The air was dark above Gravesend, and farther back still seemed condensed into a mournful gloom, brooding motionless over the biggest, and the greatest, town on earth.

The Narrator uses these rich visual descriptions to portray the Thames as a passage linking London to the open sea and then the rest of the world. He evokes a sense of infinity by calling the river an “interminable waterway” and describing the sea and sky as one vast, continuous expanse of blue. After all, when Conrad published this book in 1899, London was the world’s most powerful city, mainly because it was the capital of England’s vast colonial empire. Thus, Marlow and his compatriots are anchored in the spot that, more than any other, has shaped the global power balance they live under. This detail is key to understanding Marlow’s critique of colonialism in the novella: his country is both one of its main beneficiaries and the nation with the most power to undo it. Generations of colonial plunderers, administrators, and troops have left from the exact spot where Marlow is now warning his fellow sailors about the perils of imperial conquest. Of course, this setting also simply foreshadows the tale that Marlow is about to tell, which begins with him setting out from London for Brussels and then the Congo. 

In this opening passage, Conrad also contrasts the river’s motion (it “ran out to sea”) with the stagnation of the idle tide and the sails on the boats that represent Europeans’ colonial ambitions. This passage is also structured by the contrast between light and dark: the “luminous space” of “the sea and the sky” under the setting sun contrasts with the “mournful gloom” that “brood[s]” over London. Of course, this visual imagery foreshadows the association between light, Europe, and morality (on the one hand) and Africa and darkness (on the other) that recurs throughout the novella. By one interpretation, Conrad uses both of these contrasts to suggest that the era of European imperialism is—or ought to be—coming to an end: the setting sun, stagnant boats, and idle tide all suggest that colonialism is fading and giving way to a new world order. By another, darkness is overtaking London itself, which represents Conrad’s view that plunder, corruption, and deceit are the core reasons for England’s global imperial power and economic success. But a third interpretation is that the bright clouds in the distance could represent the Narrator’s faith in English colonialism at the beginning of the book—a faith that he will gradually lose as it advances.

This imagery of a graying Thames at sunset returns at the very end of the novella, in Part 3. But now, the Narrator’s perspective changes. Instead of seeing a shining light in the ocean and dark clouds over London, he now sees everything as dark: 

The offing was barred by a black bank of clouds, and the tranquil waterway leading to the uttermost ends of the earth flowed sombre under an overcast sky—seemed to lead into the heart of an immense darkness.

This shifting sunset imagery supports all three interpretations of the opening passage. The darkness leading out to sea could represent the Narrator’s newfound recognition that colonial conquest is morally evil (dark) rather than good (light). Alternatively, the growing cloud of gloom could symbolize Conrad’s conclusion that all of humanity is inherently selfish and corrupt, including the Europeans who often think of themselves as superior to other groups. Finally, the spreading darkness could simply represent how Europeans and their societies have spread their corrupt practices around the globe through colonialism.

Thus, while this visual imagery about light and dark clearly represents the novella’s central idea about the evils of colonialism, exactly how it does this is still up for debate. Indeed, Conrad deliberately leaves scenes like these ambiguous, so that his readers can reach their own conclusions.