From the back of the book:
The Jungle Hides Many Secrets
A vast merchant consortium has a simple job for you—one that pays extremely well. All it asks is that you stand guard over one of its outposts for a few weeks until reinforcements arrive. The problem, of course, is that the outpost is in the middle of a vast and dangerous jungle and the natives seem intent upon putting an arrow in you at every turn. If the journey there doesn’t kill you, though, you might just learn what is worth risking your lives over.
River Into Darkness is a wilderness-based adventure for 4th-level characters compatible with the world’s most popular fantasy roleplaying game. This adventure includes all the information you need to send your heroes on a fantastic journey through the heart of the deep and mysterious jungle, including encounters with terrible monsters and hostile natives.
This adventure is set in the Mwangi Expanse, the largest jungle in the Pathfinder Chronicles Campaign Setting, but it can be easily adapted for use in any setting. For more information on this environ, check out the Pathfinder Chronicles Gazetteer.