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 Croatian. 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.
After killing Rob and the unsuccessful attempt on Alison, Anna's kidnappers Mr. Kidd and Mr. Mcrae murder two nearby poachers and take their rifles, using them to open fire as Alison regroups with the others. During the attack Jenny is shot and killed, and Anna almost drowns after being pulled into river rapids, where she is eventually rescued by Alison. Alex distracts the two men by running in the opposite direction, holding a backpack wrapped in blankets that they mistake for Anna. Deciding to "follow the money", the two men give chase and gun him down, allowing Alison, Anna and Ed to escape the wilderness and reach the nearby town of Annan Mor. Meanwhile, Serbian mobster Darko, accompanied by British mercenaries Andy and Chris, travels to the area to negotiate a ransom exchange with Mr. Kidd and Mr. Mcrae on behalf of his employer.
Having been unable to recapture Anna, Mr. Kidd attempts to bluff his way through the negotiation with Darko, recalling a previous kidnapping when he murdered a young boy in Paris when his parents tried to avoid paying. Before Alison, Ed and Anna can be transported to Inverness by the police, they are tracked down by Mcrae, who kills the officers before pursuing them through the town, which is in the middle of hosting its Beltane festival. As the surviving mountaineers flee, Chris shoots Ed when he mistakes him for one of the kidnappers and is in turn shot by Mr. Mcrae, but manages to inform Darko that the kidnappers no longer have Anna before dying. After finishing off Ed, Mr. Mcrae chases Alison and Anna into a local house, which catches fire as he and Alison struggle. The fight eventually ends with Alison killing him by pushing him out of a window. She then manages to save Anna from the burning building before being rescued by firefighters. She is then transported to the hospital in an ambulance as Anna remains by her side.
Mr. Kidd nearly escapes with the ransom money, but is captured by Andy and brought before Mr. Rakovic, a Serbian war criminal who is Darko's boss and Anna's father. Rakovic has him tortured and buried alive in the woods for the kidnapping. Andy is paid the full fee for his services plus the car, with Rakovic remarking that he is in his debt.
