On a daredevil caving holiday in the Appalachian Mountains, six women friends are unexpectedly trapped underground when a rock fall blocks their exit. Searching the maze of tunnels for a way out, they find themselves hunted by a race of fearless, hungry predators, once humanoid but now monstrously adapted to live in the dark. As the others battle for their lives, Sarah, still recovering from a mental collapse brought on by the recent deaths of her family, is fighting for her sanity. When old secrets are revealed, the friends turn on one another, causing the group to implode. Betrayed and desperate, Sarah realizes that to make it back to the surface, she must become as savage as the creatures themselves.