In 1981, Yossi Ghinsberg (Daniel Radcliffe), a 21-year-old, travels to South America in the hope that he will have the adventure of his life. He arrives in La Paz, Bolivia, where he meets and becomes a friend of Marcus Stamm (Swiss professor), and Kevin Gale (Alex Russell), an American photographer. Together, the three begin on an expedition to the uncharted Amazon being led by the mysterious Karl Ruprechter (Thomas Kretschmann), an Austrian who claims to have knowledge of a hidden treasure.
After traveling together for a few days with low supplies and full nerves, the four men decide to break apart – Karl and Marcus will walk through the jungle while Yossi and Kevin will take the path of the kayaking river. But when the kayak approaches a cascade, Yossi and Kevin lose control and are in turn separate from each other, remaining isolated without supplies and little food.
Yossi, who stays at the forefront of the film’s events, is desperate to find a way back before hallucinations, lack of food, and the little drinking water he can get to take away. He rocks himself on his own for several weeks through the jungle that is now his home, and very soon he can become a place of his own. Not having the most elementary survival tools and very little knowledge of wildlife survival, Yossi has to improvise his shelter and means to buy food.