One year after a tragic accident, six girlfriends meet in a remote part of the Appalachians for their annual caving trip. Deep below the surface of the earth, disaster strikes when a rock falls and blocks their route back to the surface. The group splinters and each one pushes on, praying for another exit. But there is something else lurking under the earth -- a race of monstrous humanoid creatures that have adapted perfectly to life in the dark. 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.