After a tragic accident, six friends reunite for a caving expedition. Their adventure soon goes horribly wrong when a collapse traps them deep underground and they find themselves pursued by bloodthirsty creatures. As their friendships deteriorate, they find themselves in a desperate struggle to survive the creatures and each other.

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.