What joins together the beginning and the end of Conrad's Heart of Darkness are the repeated instances of figurative and literal darkness, supporting the theme of darkness in the story.
When the story begins, Marlow is describing his trip into the African Congo. Very much a man of the sea, he begins by sharing with his comrades a tale of one of his exploits. They listen as Marlow describes the manner in which he ended up traveling to the interior of the Congo and meeting Kurtz. As he relates what he saw and what occurred, he repeatedly refers to the darkness of the jungle. This refers literally to the depth of his journey into a region of which little is known. Figuratively, it also describes the darkness in mankind's capacity for barbarism visited upon other human beings, while considering themselves not only "civilized," but also inherently superior.
In taking a position with an ivory trading company, Marlow is sent to retrieve Kurtz: a man who is something of a marvel and celebrity. Kurtz has been inordinately successful in gathering shipments of ivory from the innermost parts of the Belgian Congo, with numbers meeting and surpassing those of any other agent of the Company.
When Marlow finally finds Kurtz, he discovers not a man of strength and power, that one might expect of a man enjoying such widespread success; instead Kurtz is insane, worshiped by the natives and very ill. He has seen horror and worse, had a hand in it as well. He dies before Marlow can get him out.
At the end of the story, Marlow is recounting the aftermath following his quest to locate and return Kurtz. In this section of the novel, he is relating a dressed up version of his interaction with Kurtz, to the dead man's fiancée—for he cannot tell her what he really witnessed. While sharing his seafaring adventures is not a new experience for him, being confronted by the grief of the woman Kurtz left behind is something with which Marlow has no skill.
In actuality Marlow's knowledge of Kurtz is very unlike that of his fiancée. While she believes Kurtz to be a man without equal, Marlow recalls:
I had a vision of him on the stretcher, opening his mouth voraciously, as if to devour all the earth with all its mankind. He lived then before me; he lived as much as he had ever lived—a shadow insatiable of splendid appearances, of frightful realities; a shadow darker than the shadow of the night, and draped nobly in the folds of a gorgeous eloquence.
I remembered his abject pleading, his abject threats, the colossal scale of his vile desires, the meanness, the torment, the tempestuous anguish of his soul...
Thinking back, Marlow recalls a cache of ivory Kurtz believed should be his and not the Company's—because it was ivory he gathered "at great personal risk"—at the time he told Marlow:
I want no more than justice...
Kurtz did not deserve justice in light of all he had done, but he demanded justice from the Company! Ironically, the justice Kurtz did deserve, the jungle finally exacted from him as Kurtz dies of a disease contracted during his extensive stay in Africa.
The finish to the story that had begun with truths has now become an exercise in saying only what the grief-stricken woman wants to hear. Marlow has no choice other than to avoid accurate details recounting the man Kurtz had become. Hardly Kurtz's friend, Marlow was with him at the end, so the woman looks to Marlow to comfort her and testify to Kurtz's greatness. In not telling the truth, the darkness of the jungle infiltrates (Marlow imagines) the drawing room where he sits. Marlow not only keeps the truth from her, but he also lies and tells her that Kurtz's last words were about her.
As his fiancée continues to praise Kurtz, Marlow begins to become irate:
"His end," said I, with dull anger stirring in me, "was in every way worthy of his life."
This is one of the few truths Marlow can share.
The beginning and end of the story are mostly strongly linked by the repeated images of darkness threaded throughout Conrad's story. The images of darkness from the story's onset are now present at the end for different reasons. Presented first is the darkness of the jungle that is overgrown and allows little light to enter. What follows is the worst of the darkness, found in Kurtz's soul. Distorted echoes of this inner darkness reverberate around him as Marlow lies about Kurtz's character and his last moments of life. Marlow notes that the heavens did not fall down around him for doing so; but...
Would they have fallen, I wonder, if I had rendered Kurtz that justice which was his due? Hadn't he said he wanted only justice? But I couldn't. I could not tell her. It would have been too dark—too dark altogether...
The heart of the darkness Marlow witnesses on his journey lies not only in the darkness of the jungle's interior, but also in the hearts of so many of the men and situations Marlow faced. Ultimately, he finds that the darkness follows him back to the mainland, sneaking into the home of the woman who mourns Kurtz, where the light of truth remains hidden.