Idle Morning Thoughts on Darkness
I'm sure it's been said before, but Marlow is Conrad's superego which goes in to find Kurtz, his id. He finds him, but in finding him, kills him, destroys him; the id cannot survive outside its lair in the unconscious, the heart of darkness--Africa to Europe's supergo, to extend the conceit. 

Thus, Marlow's narrative within the brief framing story, which is itself a first-person account, is the content of a case study. The first-person narrator, a stand-in for Conrad, is the analysand. Like Conrad himself, Kurtz is a product of many nations: "All of Europe went into making Kurtz." The reader is the analyst. 

The death of the id is why we cannot see Marlow; he has disappeared with the death of his necessary Other, and this is why the narrator is so effaced: psychologically, they are both absented by their lack of an id. The text is their account of how they became a psyche incomplete.