The novel begins with Marlow telling his story to others aboard a ship on the Thames river. This location causes Marlow to ruminate on imperialism as a universal attribute of powerful nations. Marlow imagines the Romans coming into Great Britain and having to deal with the conquest of a "savage" people, just as the British and other Europeans in his era, the nineteenth century, have dealt with the conquest of "savage" people in Africa. The Romans, Marlow says, setting a frame for understanding nineteenth-century imperialism, simply came to Great Britain and performed
robbery with violence, aggravated murder on a great scale...
That more or less sums up European imperialism in Africa as well.
The novel then goes on, not surprisingly, to critique nineteenth-century imperialism for not living up to its claims of bringing order and civilization to Africa. In fact, the system Marlow finds in Africa is chaos. The continent is simply being exploited by the Europeans for the wealth it can garner them. The great exemplar of this system run amok is Kurtz. He brings in the most ivory of any station manager in the Belgian Congo, which makes him a valuable asset to the Europeans; but he achieves these goals through corrupt means and goes insane, insisting the natives worship him as a god. While Marlow, in some ways, admires him for the purity with which he understands imperialism, he is the symbol of how imperialism is evil and brings unspeakable violence, destruction, and pain to native peoples.
Marlow addresses British imperialism as a great evil: one that has to be hidden from such innocent symbols of civilization as the women left at home, who are protected from true knowledge of what is going on so that the illusion that the Europeans are a civilizing influence can be preserved.