Anja and five friends join anthropology student Dace on a journey to study a remote ancient rock painting. Their excitement vanishes when Mel becomes delirious after skinny-dipping in the waterhole. Feverish bleeding and confused she physically and mentally regresses to a vicious predatory state. Mel has gone primal. Mel's lover and friends realize they are the prey as she savagely hunts them down. Before they can escape one of them starts to regress posing a hideous choice: kill their friends or be killed by them. Their only hope of survival is through a cave where Anja learns too late the meaning of the ancient rock art they came to study.