Mountaineers Alison, Ed and Rob meet up with friends Jenny and Alex for a climbing and hiking trip in the Scottish Highlands. Whilst taking a break for lunch, they discover Anna, a young girl buried alive in a small chamber in the wilderness. They are unable to communicate with her as she speaks no English, with Ed guessing she might be Croats. Deciding they need to get her to safety, Alison and Rob elect to take a shortcut to the nearest village to fetch help, but the route involves having to abseil down a high cliff named "Devils Drop". During their descent, Rob's rope apparently breaks, causing him to fall to his death. Alison is pelted with falling debris as she tumultuously reaches the bottom, whereupon she discovers Rob's rope was cut. Seeing a figure move away from the top of the cliff, it becomes clear the people who imprisoned Anna are trying to kill them.