It is important to remember that this incredibly profound novella is structured around what is termed a frame story - that is there is a story within a story (the tale of Marlow's journey into the heart of darkness) which is told as a separate story to the listeners on the Nellie who we are introduced to at the beginning and return to at the end of the novella. Interestingly, at various moments, Marlow addresses his audience in the middle of his narration as well.
The story begins then as Marlow and four others are gathered awaiting for the turn of the tide so that they can leave where they are left. Interestingly, the audience of the tale Marlow is about to tell are named by their function: the Director of Companies, the Lawyer and the Accountant. This makes us think of them as a carefully selected audience representing the various aspects of colonialism. As for the landscape, we are given a description:
The sea-reach of the Thames stretched before us like the beginning of a n 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 the 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 mood that is created is at once meditative and reflective and also foreboding, with reference made to the "mournful gloom." What is key to note, however, is that by the time Marlow has finished his story, the darkness has shifted position:
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.
Note how, symbolically, after hearing Marlow's tale, the darkness covers the waterway and the way out to the sea and colonial lands, suggesting something very profound about the darkness and evil inherent in colonialism.