Anthropological professor Conrad Hamilton attempts to study a new species of primate, possibly the missing link between humanity and the great ape, found in a hidden valley deep within the jungles of Thailand. Hamilton's initial research team tries to capture one of these new (and very large) primates, but fail and are all killed. Hamilton and his assistant Chenne, who survive because they are away from the camp site, scour the area looking for clues and remains of their team.
Meanwhile, another research team is inbound, this one a crew of college anthropology students with no idea of what they're in for. The students, Seth, Amy, Greg, Sydney, Josh, and Dani, are flown into a remote region of the Thai jungle, and picked up by a guide who drives them deeper into bush. He drops them off in a panic at the edge of trail/road, which leads further still into the foliage, claiming "bad things" are in there and won't go any further. He heads back the way he came, leaving the students to march forth into the unknown. They walk until they reach the end of trail and set up camp. As evening sets in, noises from the jungle raise suspicion until a set of glowing green eyes can be seen close by, watching. Just before the unknown creature attacks, Chenne arrives with a flare that scares off the unseen menace.
Chenne escorts the students to the relative safety of Professor Hamilton's camp, and the following day they meet the obsessed man and somewhat learn of his mission and their purpose. Hamilton professes of dream findings in an uncharted valley located deep the jungle and their potential for career-launching documentation. He has Chenne confiscate their mobile phones and hand out information bracelets for each member that contain all of their emergency contact info, then he leads the slightly unwilling team to the valley entrance. After a pep talk, Hamilton convinces the students to continue and repel down the cliffside and into the valley, although Josh is injured during the process.
On their first night in the valley, Hamilton passes around a skull and explains that it's creature he's looking for. The students cannot identify the skull since its characteristics are both human and primate in nature, but nearly twice the size of any known human or primate cranium. They are soon interrupted by a bloody survivor from the original research team, who Hamilton and Chenne quickly shelter and care for. The man dies shortly afterward, and Hamilton tells Chenne that the creatures let him go as a warning.
During the night, Sydney visits the outhouse, only to be dragged away into the jungle. The next morning, Hamilton tells the team that Sydney came to him scared and homesick and wanted to go home, so Hamilton had Chenne take her out of the junglle, leaving everyone suspicious. However, in another part of the jungle, Chenne is dragging Sydney through the brush and eventually leaves her, battered and beaten. Sydney eventually stumbles into an unseen creature that tears the right side of her face off.
The team continues to follow Hamilton and Chenne, who appear to be tracking something with a GPS reader. Unknown to the students, Hamilton is tracking each of the students by a hidden chipset in each of their bracelets. At the moment, he is tracking Sydney's bracelet. The team eventually demands more information about their expedition, and Hamilton comes mostly clean. The students remain unaware that they are being tracked, but resolve to steal the AK-47 in Hamilton's possession and maintain control of the situation to themselves. Seth begins leaving a trail through jungle by tying off pieces of cloth to trees.
On their third night, while the students are beginning to fashion their plan for the following day, a foul-smelling rain begins pouring down on their tents. Just as they recognize the smell as urine, Josh is yanked out of his tent and dragged up into the trees. Panic sets in as the team scatters and begins following his screams through the jungle. As Greg attempts to save Josh, Chenne accidentally shoots him, but then proceeds to tie him to a tree as bait. She camps out nearby with her gun, but one of the monsters sneaks up on Chenne and kills her before mauling Greg.
Hamilton runs across Seth during the chaos and knocks him unconscious after listening to his complaints. Hamilton locates the two remaining students, Dani and Amy, and continues his venture, ordering Dani to document everything with her video camera. Believing themselves to be the only survivors, both girls have little choice other than to follow the professor in hopes of being rescued. The professor successfully tracks Sydney's bracelet and finds it still attached to her severed arm which is dangling from a tree. Hamilton examines an apparent rigging done to the tree, only to spring a trap which results in several bamboo shoots impaling him through the back. He delivers a final address to the girls before he dies.
Just as they turn to run, Seth appears, to their delight. They run back to their campsite, only to find it cleared of their tents and equipment. Completely panicked, they keep running while the creatures seem to be following close by in the brush and in the trees. Dani is soon pulled up into the trees and killed, leaving Seth and Amy. They run further still and reach a cave, where they see Seth’s entire cloth trail assembled and attached to the opening. They go into the cave and use a the night vision from Dani's video camera to move around. At least one of the creatures follows them into the cave, grabs Seth and kills him.
Amy sheds light on one of the creatures for the first time, revealing it to be a huge gorilla with a bloody set of fangs. Several more gorillas enter the cave and Amy screams in terror as one of them bears down her and kills her