Marlow's story, which makes up the actual plot of Heart of Darkness, begins with backstory about his passion for travel and desire to pilot a steamboat. While on his journey into the African interior, he receives his first exposure to the horrors of the jungle and the contrast with European imperialism:
...all about others [native people] were scattered in every pose of contorted collapse, as in some picture of a massacre or a pestilence.
When near the buildings I met a white man, in such an unexpected elegance of get-up that in the first moment I took him for a sort of vision.
(Conrad, Heart of Darkness, eNotes eText)
It is this contrast that gives Marlow the first idea that something is wrong with his preconceived notions. As it turns out, his original mission will be superceded by the Company's desire to retrieve Kurtz, who has vanished into the jungle; the only reason they know he is alive is that he is still sending out ivory. Marlow slowly becomes aware that the Company is simply exploiting the jungle and its peoples, instead of "civilizing" them; Kurtz represents the worst of their excesses, destroying all around him while pretending that he is a great constructor.