The story begins with a couple camping in the Scottish Highlands. The woman gives the man a silver letter opener as a present, soon after they are attacked in their tent. Meanwhile, Private Cooper is seen running through a forest in North Wales. He attacks his pursuers, but is overwhelmed and wrestled to the ground. It turns out Cooper was trying to join the special forces, but fails when he refuses to shoot a dog. He is returned to unit by Captain Richard Ryan.
Four weeks later, a squad of six regular British Army soldiers, including Cooper, is dropped into the Scottish Highlands. Expecting to carry out a routine training mission, they find the savaged remains of a special forces squad who were part of the same training exercise. The single survivor, Captain Ryan, makes cryptic references to what attacked them. It is noticed that he has been wounded. Unseen antagonists make their presence known as they attack the troops. Whilst retreating, Bruce is impaled on a tree branch and Sergeant Wells is attacked. He is rescued by Cooper and carried to the roadside where the group encounter Megan, a zoologist who takes them to a lonely house. The soldiers that remain are Wells, Cooper, Spoon, Joe, and Terry. As darkness arrives, the house is surrounded by the attackers: to the soldiers' incredulity, these are revealed to be werewolves.
After giving Wells treatment, they go to get in the car, but it was destroyed by the werewolves. The soldiers maintain a desperate defense against the werewolves, believing that if they can make it to sunrise, the werewolves will revert to human form. After Terry is abducted and ammunition runs short, they realize that they will not last, and decide try and escape. Spoon creates a distraction while Joe steals a Land Rover from the garage. When he gets in the car, he sees Terry in the garage being eaten by a werewolf. The werewolf rips off Terry's head and throws it at the windshield. Joe drives up to the house door, then realizes that a werewolf was hiding in the back seat. Joe is killed. Ryan transforms into a werewolf due to his previous wounds, but not before revealing in an angry exchange with Cooper that the Government had sent him on a mission to capture a live werewolve in order that they could be investigated and weaponized. A fight ensues and Ryan escapes, running off into the forest. The soldiers try blowing up the barn where Megan tells them the werewolves must be hiding with petrol, gas canisters, matches, and the Land Rover. Once it's been destroyed, Megan reveals that not only were there no werewolves in the barn, she only told them that to destroy their only means of transportation; she is a werewolf as well, and had been suppressing the transformation, but now gives in. Cooper and Wells run upstairs and Spoon runs to the kitchen, blocking the door. The werewolf breaks in and Spoon's rifle jams. He fights the werewolf, but is killed by another. Meanwhile, Wells and Cooper shoot through the floor upstairs to escape the werewolves, dropping into the kitchen. Wells cuts a gas line and blows up the house whilst Cooper hides below in the cellar. Before he can flee, Ryan, transformed into a werewolf, confronts him. Cooper kills him with the silver letter opener and shoots him in the head. He and Megan's dog, Sam, walk off into the woods.