Blood Monkey is a horror / jungle suspense thriller set in modern day Thailand. The film centers around anthropological professor Conrad Hamilton (F. Murray Abraham) attempting 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. The movie opens into action with Hamilton's initial research team trying to capture one of these new (and very large) primates, but fails and is apparently wiped out. Hamilton and his assistant Chenne (Prapimporn Karnchanda) survive and scour the camp site looking for clues and remains of their team.
Meanwhile, another research team is inbound, this one a motley crew of incredibly naive 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 high-tails it 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 learn of his mission and their purpose...sort of.
Hamilton professes of dream findings in a uncharted region of the jungle and their potential for career-launching documentation. He has Chenne confiscate their mobile phones and hands out information bracelets for each member, which contain all of their emergency contact info, and promptly 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. 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, and Hamilton claims the creatures let him go, as a warning. During the night, Sydney (Laura Aikman), visits the outhouse, only to be dragged away into the jungle. The next morning, Hamilton tells the team he had Chenne escort Sydney back out of the jungle, leaving everyone very suspicious. However, in another part of the jungle, Chenne is dragging Sydney through the brush and eventually leaves her, a bit battered and beaten. Sydney stumbles into an unseen creature and screams as the right half of her face is promptly bitten 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 clean...mostly. The students remain unaware that they are being tracked, but resolve to steal the AK-47 in Hamilton's possession and leverage control of the situation to themselves. Seth begins leaving a breadcrumb 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 (Sebastian Armesto) is dragged up into canopy of the trees. Panic sets in as the team scatters and begins following his screams through the brush. As Greg (Matt Reeves) attempts to follow Josh, Chenne accidentally shoots him, but then proceeds to tie him to a tree as bait. Chenne camps out nearby with her rifle while Greg howells in pain. The still unseen primate sneaks up on Chenne and kills her, just before charging the helpless Greg and dismembering him.
Hamilton runs across Seth (Matt Ryan) 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. Believing themselves to be the only survivors, both women have had more than enough, but have little choice other than to follow the professor in hopes of being rescued. The professor successfully tacks Sydney's bracelet and finds it still attached to her amputated arm, which is dangling from a tree. The professor examines the apparent rigging done to the tree and the arm, only to spring a trap which results in several bamboo shoots protruding from his chest from the back. He delivers a final address to Dani's video camera as they watch him die. Just as they turn to run, Seth appears, battered but alive, to their extreme delight. They run back to their campsite, only to find it cleared of their tents and equipment. Completely panicked, they keep running, all the while the creatures seem to be following close by in the brush and in the trees. Dani is soon swept up into the canopy from the back of the threesome, leaving Seth and Amy with her camera. They run further still and reach the mouth of the cave, where they see all of Seth's cloth trail assembled and attached to the opening.
They of course, go into the cave, and use the night vision on the camera to move around. At least one of the creatures follows them into the cave, and grabs Seth in the final moments. The videocamera drops to the floor, and sheds light on one of the creatures for the first time in the film, revealing it indeed to be a huge gorilla with a very bloody set of fangs. Amy is clearly the last victim and she screams as the gorilla bears down her to end the the final scene. No happy ending. What happens in Thailand, stays in Thailand.