A shipboard narrator records a companion's account of a perilous inland expedition in search of wealth, describing travel through a lush, uncanny tropical interior, interactions with local peoples, and the mounting strain on the party's nerves. Geological and collecting work draws them to the slopes of a forbidding peak, where natural hazards, cultural clashes, and ambiguous illnesses produce eerie incidents and psychological unraveling. The narrative alternates seafaring framing scenes with episodic adventure and meditative passages on physiological and mental disturbance, shifting the tone from exploration to growing apprehension.