After Indiana Jones and Mutt Williams had found the Crystal Skull of Akator in Orellana's grave in Peru, they were captured by Soviet soldiers. They were flown to Iquitos, continued by ship on the Amazon River to Ilha Aramacá, and were then taken to the jungle camp where they were reunited with Harold Oxley and Marion Ravenwood.
From a series of ideograms that Oxley had drawn on a sheet of paper, both Jones and Spalko learned that they had to follow the "sleeping water" (Sono River) until it reached the "great snake" (Amazon River) to find Akator where the skull was to be returned.
During an attempt to flee from the camp, Jones and Ravenwood got trapped in a dry sandpit. While Mutt succeeded in getting them free using a rat snake as a rope, a disoriented Oxley, in his attempt to get "help", brought the Soviets so that they all were recaptured.
Behind the scenes
Even though the jungle camp is the first location shwon in the movie after the progressing red line on the map has reached Ilha Aramacá, it remains open whether the camp really is located on or near that island. While the camera zooms into the camp, a river can bee seen in the background, but it is not clear whether this is the Amazon River. It would be difficult to understand how they the should follow the Sono River until it reaches the Amazon River if the camp was already at the Amazon River.