One year following a horrendous and tragic accident, six girlfriends meet for their yearly adventure in a remote part of the Appalachians for a caving trip into the arteries of the earth. Deep inside the caves, disaster strikes again when a rock fall blocks their route back to the surface. The girls learn that Juno, the leader of the expedition who is always pushing herself that bit further, has brought them to an unexplored cave and that no one is coming to rescue them. The group splinters and each push on, praying for another exit. But, there is something else lurking under the earth - a race of monstrous humanoid creatures that are adapted perfectly to life in the dark. As the friends realise they are the prey, they unleash their most primal instincts in an all-out war against an unspeakable horror - one that attacks without warning, again and again and again.