The 8th Plague is the story of a woman named Launa who is investigating the disappearance of her sister, Nikki. Nikki had gone camping in the mountain town of Halcyon Springs and never came back. Becoming more and more frustrated, Launa realizes that she must find her sister on her own with the help of Mason, a mysterious local man with a shadowed past. Their search leads them to an abandoned prison called Halcyon Ridge Correctional Facility. Very soon after arriving within the stone and steel walls they are exposed to an ancient evil that had been lying dormant until its recent disturbance. Subsequently they learn the terrible truth about what happened to Nikki as well as the impending plague of darkness that intends to spread and reclaim the world.

The crew of a U.S. Navy submarine in World War II rescues survivors from a sunken ship, only to face a series of mysterious deaths and supernatural occurrences.