A caving expedition goes horribly wrong, as the explorers become trapped and ultimately pursued by a strange breed of predators.

In the wake of the sexual revolution and the rise of the women's movement, the 1973 tennis match between women's world champion Billie Jean King and ex-men's-champ and serial hustler Bobby Riggs became one of the most watched televised sports events of all time, reaching 90 million viewers around the world. As the rivalry between King and Riggs kicked into high gear, off-court each was fighting more personal and complex battles.

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....

Distraught, confused, and half-wild with fear, Sarah Carter emerges alone from the Appalachian cave system where she encountered unspeakable terrors. Unable to plausibly explain to the authorities what happened - or why she's covered in her friends' blood – Sarah is forced back to the subterranean depths to help locate her five missing companions.