Professor Hamilton F. Murray Abraham is an anthropological researcher who travels to Thailand to study a newly observed primate, hoping to uncover a missing link between humans and great apes. His expedition quickly turns perilous when the rest of his team dies during a dangerous attempt to capture a specimen for closer examination, leaving him with his assistant, Chenne Prapimporn Kanjunda as the sole survivor.
Elsewhere, a group of college anthropology students is flown into a remote jungle region and collected by a wary guide who refuses to take them deeper, warning that “bad things” lie beyond the edge of the trail. At the end of the road, the students set up camp, and during their first night in the valley, they are attacked by a creature. Chenne intervenes, fending it off with a flare and guiding the students toward Hamilton’s camp.
The next day, Hamilton speaks of an uncharted valley that could yield career-defining footage, and he orders Chenne to confiscate the students’ phones while distributing bracelets containing emergency contacts. He then leads the group down into the valley, where they rappel down a cliff and injure Josh Dawson Sebastian Armesto along the way.
On the first night among the trees, Hamilton shows the group a skull—enormous, unlike any known human or primate cranium—and the mystery deepens when a bloody survivor of the original expedition appears and dies under their care. Hamilton believes the creatures may have sent a warning by sparing the man’s life.
Sydney Maas Laura Aikman is the first to encounter trouble, slipping away to an outhouse only to be dragged into the jungle. The following morning, Hamilton claims Sydney returned frightened and asked to leave, so Chenne escorts her out of the jungle. But Chenne drags Sydney back into the wilderness and leaves her battered, and Sydney ultimately confronts a creature that kills her.
As the team presses on, Hamilton remains a step ahead, secretly tracking every student via the bracelet chips. The students hatch a plan to take control by stealing Hamilton’s AK-47, while Seth Matt Ryan begins marking a path through the jungle with pieces of cloth tied to trees to guide their escape.
One night, a foul-smelling rain—really a stream of urine—pummels the tents. Josh is dragged from his shelter and raised into the trees, and the others scatter, chasing his screams through the dark. In the confusion, Chenne shoots Greg Satch Matt Reeves by accident and then leaves him tied to a tree as bait. She camps nearby, but a monster stoops him, and later a second assault claims her life as well.
Hamilton encounters Seth amid the chaos and knocks him unconscious before locating the two remaining students, Dani Sudeva Freishia Bomanbehram and Amy Armstrong Amy Manson. He forces Dani to document the calamity on camera while he continues to hunt for Sydney. Tracing Sydney’s bracelet to a severed arm dangling from a tree, Hamilton discovers an elaborate trap that drives bamboo shoots through his back, and he dies.
With their campsite emptied of tents and supplies, the two surviving students—Dani and Amy—flee deeper into the wild, convinced they are the only ones left. They reach a cave where the path they’ve followed is laid out, and Seth is killed inside. When Amy finally turns a light on one of the creatures, the monster’s true form is revealed: a large, blood-streaked ape. More of the primates emerge, and Amy’s terrified scream ends as she is attacked and killed by one of the creatures.
