An ancient enemy of humanity returns! In Croatia, cave paintings are discovered depicting humans fighting against giant shadowy creatures. However, before the archaeologists can investigate further, their camp is attacked. At the same time, a primate research station near Atlanta is raided. The search for connections takes Commander Grayson Pierce of the Sigma Force 50,000 years into the past and around the globe - to an ancient city of the dead: the Bone Labyrinth.