Nursery school teacher Jenny (Kelly Reilly) and her boyfriend Steve (Michael Fassbender) embark on a romantic getaway to a picturesque, secluded lake nestled in the lush English countryside. En route, they encounter a young boy who seems hesitant to engage with them. Upon their arrival at the beach, they hope for a day of relaxation, but their plans are swiftly disrupted by a rowdy group of teenagers accompanied by an unruly dog. Despite the chaos, Steve stubbornly opts to stay put, choosing to endure the loud music and disruptive antics rather than let the teens "intimidate" them into leaving.
As the sun sets, the couple camps out in a tent, only to be met with dismay when they discover their food has been infested with insects the following morning. In search of a meal, they head back to their car, only to accidentally run over an empty bottle discarded by the hooligans. Steve manages to change the tire, and they venture into town for sustenance. In a diner, Steve inquires about the troublemakers, laughing off their antics, but the waitress defensively claims, "Not my kids."
Refusing to be dissuaded, Steve spots bicycles outside a nearby house and, against Jenny's pleas, enters the property in pursuit of answers. After a close call with the owner, who is the father of one of the teens, Steve narrowly escapes and rushes back to the lake. Later, while Jenny dozes on the beach, Steve goes scuba diving. When he returns, they quickly realize that their beach bag containing car keys, Steve's phone, and wallet has gone missing. Their worries escalate when they discover their vehicle is also gone, leaving them no choice but to trek back to town on foot. As they make their way, they narrowly avoid an oncoming car piloted by the teens, with Brett (Jack O'Connell), their leader, smirking at them.
As night falls, they unexpectedly encounter the gang in the woods. Steve confronts the delinquents against Jenny's wishes, demanding the return of their items. However, Brett denies involvement with the theft. When Brett's phone rings from his pocket, a scuffle ensues, resulting in Steve accidentally fatally injuring Brett's dog. Grieving yet furious, Brett tosses the car keys at the couple and demands they leave. Although they successfully recover their vehicle, a steep slope causes them to become stranded. Soon after, the gang finds them and begins hurling rocks at the car until they crash into a tree.
In a harrowing twist, Steve asks Jenny to escape as he struggles to free himself from the wreckage. She flees into the woods, seeking refuge for the night. The next morning, she returns to find Steve missing and discovers a trail of blood leading to the gang, who have captured him and tied him with barbed wire. Witnessing the horrifying torture inflicted on Steve, Jenny uses the Bluetooth feature on his GPS to connect with his phone, drawing attention to her location.
Panic-stricken, Jenny runs as the gang chases her. This creates a chance for Steve to break free from his constraints. After a desperate search, the couple reunites, but their relief is short-lived as they find themselves cornered by the gang once again. They narrowly escape when Jenny eludes capture by hiding in a water reservoir, but during this time, Steve is gruesomely murdered by the group.
In a chilling moment, as Jenny attempts to seek help, she encounters Adam (James Gandhi), the boy they had met earlier, only to be betrayed. As she regains consciousness later, Jenny realizes the gang plans to burn her and Steve alive. In a frantic escape, she manages to free herself just as they light the fire; however, their revenge turns gruesome as they threaten Adam, leading to his tragic death.
Fueled by desperation, Jenny finds a map to navigate her way out of the nightmare. After coming up against the gang once more, she fights back and inadvertently fatally wounds Cooper ([Thomas Turgoose]), one of the gang members, spurring chaos among the group. Unable to ignore the scene's violence, frightened by Brett's rage, the gang members begin to turn on each other.
Eventually, Jenny makes her way to the road, flagging down a van and pleading for refuge. However, she soon finds herself in Brett's family home, where she discovers that the gang's parents, complicit in their children's terror, are equally dangerous. As Brett returns, he unleashes their wrath on Jenny, and the film concludes with her screams echoing, leaving her fate uncertain amid the violent chaos of familial betrayal and horror.
