A squad of British soldiers, led by Sergeant Harry Wells, is dropped into the Scottish Highlands for what they believe to be a routine military training exercise. The team, including Private Cooper who recently failed a Special Forces selection because of his moral convictions, starts their mission with lighthearted banter and tough camaraderie. Their light mood shifts dramatically when they stumble across the savaged remains of another military unit, with only one severely traumatized survivor. The discovery is unsettling, and the men are beset by confusion and fear as they encounter a series of strange phenomena suggesting that they are not alone in the desolate wilderness.

As night falls, the soldiers are forced to seek refuge in an abandoned farmhouse when they are attacked by unknown assailants. Their enemies are fast, vicious, and seem almost supernatural in their abilities. In the besieged farmhouse, the squad meets Megan, a zoologist who provides them with insight into what they're up against. Tensions mount within the group as they struggle to understand their situation and find a way to survive. The soldiers must adapt and work together, utilizing their training and instincts to fight against an enemy beyond their worst nightmares. In the face of utter terror, the line between humanity and the primal instincts of survival begins to blur.