A couple is camping in the Scottish Highlands. The woman gives the man a silver letter opener as a present; shortly afterwards they are killed in their tent by an unseen assailant.
Meanwhile, Private Cooper (Kevin McKidd) 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 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 (Liam Cunningham).
Four weeks later a squad of six regular British Army soldiers, including Cooper, is dropped into the Scottish Highlands, somewhat fed up about missing an England–Germany World Cup match. Expecting to carry out a training mission against a Special Air Service (SAS) unit, they only find their savaged remains. A badly-wounded Captain Ryan, the only survivor, makes cryptic references to what attacked them. Unseen predators attack the troops. While retreating, Bruce (Thomas Lockyer) is impaled on a tree branch, killing him, and Sergeant Wells (Sean Pertwee) is attacked. He is rescued by Cooper and carried to a rural roadside where the group encounters Megan (Emma Cleasby), a zoologist who takes them to a lonely house, belonging to an unknown family. The soldiers who remain are Wells, Cooper, Spoon (Darren Morfitt ), Joe (Chris Robson) and Terry (Leslie Simpson). As dark falls, the house is surrounded by the attackers: werewolves!