At the beginning of Heart of Darkness, Marlow shares his fascination with rivers as he converses with his companions. There are five men on board a ship on the Thames. They are the Director of Companies, the Accountant, the Lawyer, Marlow, and the unnamed narrator.
As the sun sets, Marlow muses about the adventures he has had the privilege of enjoying. All of them seem to involve rivers and oceans. The narrator tells us that Marlow is the only man among the group who still "follows" the sea. In fact, the narrator takes pains to tell readers that Marlow is both a seaman and adventurer, someone who revels in the journey as much as the process of discovery.
As Marlow speaks, we learn that he was fascinated with maps during his boyhood years. He was especially curious about the rivers and lakes. One, in particular, stood out to him: the Congo.
Marlow recalls thinking that the Congo resembled an immense snake whose body lay uncoiled across a vast expanse of land. He imagined that the snake's tail was buried deep in the heart of that land.
As he thought about the river, the idea came to Marlow to captain a steamboat. He asked among his acquaintances for word about such a position. In the end, it was through the influence of an aunt that Marlow received his appointment from a trading company.
Marlow relates that his appointment was swift, as the "Company" had recently lost one of its captains, Fresleven. The Danish captain was killed after getting into a quarrel with the natives. Despite the tragedy, Marlow wasn't deterred from his dreams of traveling up the Congo. He tells us that the river beckoned to him as a "snake," deadly but fascinating.