The lonely forest ranger Danielle St. Claire works alone in Tower 19 of the Forestry Department of a remote National Park. Danny is grieving the loss of her best friend Julie Cassidy and is tormented by the feeling of guilty, since she was responsible for the death of her girlfriend. While driving Julie's car after drinking in a bar, she had an accident but she did not assume that she was driving. When a group of students releases a demon trapped in a mystical cave, the creature attacks tourists nearby her tower and later her boyfriend Justin Rawley. Without communication or means of transportation, Danny fights to survive while the monster is attracted by her sadness.