Britain, A.D. 117. Quintus Dias, the sole survivor of a Pictish raid on a Roman frontier fort, marches north with General Virilus' legendary Ninth Legion, under orders to wipe the Picts from the face of the Earth and destroy their leader, Gorlacon.

After a personal tragedy, Sarah joins her friends on a caving expedition in the Appalachian Mountains. But when a rockfall traps them deep underground, their adventure turns into a nightmare. As they search for a way out, the group discovers they are not alone—lurking in the darkness are savage, cave-dwelling creatures. With rising tension and dwindling trust, the women must fight to survive against both the predators and each other.