A couple goes camping in the Scottish Highlands. The woman gives the man a silver letter opener as a present; shortly afterward they are killed in their tent by unseen assailants. Meanwhile, a soldier named Cooper runs through a forest in North Wales. He attacks his pursuers, but is overwhelmed and wrestled to the ground. It is revealed that Cooper is trying to join a special forces unit but fails when he refuses to shoot a dog in cold blood. He is returned to his unit by Captain Richard Ryan.
Four weeks later, a squad of six British soldiers, including Cooper, are dropped into a remote area of the Scottish Highlands to carry out a training exercise against a Special Air Service unit. The following morning, they find the SAS unit's savaged remains. A badly wounded Captain Ryan, the only survivor, makes cryptic references to what attacked them. The troops retreat when unseen assailants begin pursuing them.
While retreating, Corporal Bruce is impaled by a tree branch, which kills him, and Sergeant Wells is attacked. He is rescued by Cooper and carried to a rural roadside where the group meets Megan, a zoologist who takes them to a lonely house belonging to an unknown family. Soldiers Wells, Cooper, Spoon, Joe, and Terry remain.
As darkness falls, the house is surrounded by attackers who are revealed to be werewolves. The survivors try to get in the Land Rover but find it has been destroyed by the werewolves. The soldiers maintain a desperate defence against the creatures, believing that if they can make it to sunrise, the werewolves will revert to human form.
Cooper and Megan treat Wells' wounds. After Terry is abducted and their ammunition runs short, they realize they will not be able to hold out and decide to try to escape. Spoon creates a diversion while Joe steals a Land Rover from the garage. Joe drives up to the house door, but is then killed by a werewolf that was hiding in the back seat.
Under interrogation, Ryan reveals that he was sent on a government mission to capture a live werewolf for study and exploitation as a weapon; Cooper's squad was supposed to be the bait, and considered expendable. An enraged Wells and Cooper attempt to kill Ryan, but he transforms into a werewolf and escapes into the forest. It is revealed that the Uath family, the owners of the house, are the werewolves.
The soldiers try blowing up the barn - where Megan told them the werewolves must be hiding - using the Land Rover and petrol to create a bomb. Once the barn has been destroyed, Megan reveals that not only were there no werewolves in the barn, but she is part of the Uath family and a werewolf as well. She also reveals that she unlocked the back door to the house, allowing the other werewolves to get inside. Before she fully transforms, Wells shoots her in the head. He and Cooper run upstairs, while Spoon fights a werewolf in the kitchen. Using his surroundings to his advantage, he gains the upper hand, but is eventually killed when a second werewolf intervenes.
Wells and Cooper shoot through the floor upstairs to elude the werewolves and drop into the kitchen, where they find Spoon's remains. As he begins to transform into a werewolf, Wells orders Cooper to take shelter in the cellar and gives him a roll of photographic film (which was in a flash camera used to stun the werewolves) to prove what has happened. The werewolves break into the kitchen and confront Wells as he cuts a gas line and blows up the house, killing the werewolves and sacrificing himself.
As the sun rises, Cooper attempts to leave, but the werewolf Ryan confronts him. After a brutal fight, Cooper stabs Ryan in the chest with the silver letter opener, weakening him enough to allow Cooper to shoot him in the head. Cooper, along with Megan's Border Collie Sam, emerges from the cellar. Cooper's story, with photographs, is shown reported only in a sensationalist tabloid newspaper with title "Werewolves ate my platoon!", under the results of the England vs. Germany football match.
