One year after a tragic accident, six girlfriends meet in a remote part of the Appalachians for their annual extreme outdoor adventure, in this case the exploration of a cave hidden deep in the woods. Far below the surface of the earth, disaster strikes, and there's no way out. The group splinters and each woman pushes on, praying for another exit. But there is something else lurking under the earth. As the friends realize they are now prey, they are forced to unleash their most primal instincts in an all-out war against an unspeakable horror - one that attacks without warning, again and again, and again.