Nursery school teacher Jenny (Kelly Reilly) and her boyfriend Steve (Michael Fassbender) escape for a romantic weekend at an idyllic remote lake somewhere in the green English countryside. They meet a young boy along the way who is reluctant to talk to them. When they arrive at beach they attempt to have a relaxing day, but a group of teenage delinquents and their dog appear. Steve stubbornly decides that he would much rather stay put, enduring their loud music and obnoxious dog, than be "bullied" by them into leaving.
They stay overnight in a tent, and in the morning discover that their food has been infested with insects. Hungry, they return to the car to find someplace to eat, but unfortunately, they run over an empty bottle the hooligans left behind. Steve luckily manages to fix the tire, and they set off for the town. While in town, Steve sees the unruly teens riding around on their bicycles, and angrily follows them. Unable to keep track of them, Steve finally gives up, and they go looking for some breakfast. In they diner they settle for, Steve asks the waitress if she has seen the kids riding around on bikes. The waitress jokingly asks if they have been terrorising Steve and Jenny. Steve laughs, and states they have only been "hassling" them, and continues by saying that he wanted to inform their parents they had punctured his tyre. The waitress then becomes strangely defensive, saying "Not my kids".
When they leave, though, he catches sight of a group of bicycles sprawled out on the front lawn of a house, and stops the car. Jenny pleads for him to stay, but Steve determinedly gets out. After receiving no answer to his calls at the front door, Steve enters the house, but almost gets caught by the owner - the father of one of the teenagers - who returns. Steve manages narrowly to escape out of the window, and he and Jenny hastily head back to the lake.
Once back, Steve goes scuba diving whilst Jenny sleeps on the beach, but when Steve returns they realize the beach bag, containing the car keys and Steve's phone and wallet, has gone missing. Hurriedly, they go to check on the car, and much to their dismay, see that it is gone as well. Making their way back to town on foot, they narrowly avoid being hit by the car as the gang drives it wildly through the woods, stopping only for Brett (Jack O'Connell) - the leader - to smirk at the couple.
Later at nightfall, Steve and Jenny come across the gang in the woods. Steve, against Jenny's will, confronts them. He demands the car keys and his possessions back, but Brett denies any knowledge of stealing their camping equipment or their car. Unfortunately for him, the phone goes off in his pocket, prompting a scuffle, during which one of the boys produces a knife. Steve manages to grab the knife, but in the confusion ends up stabbing - fatally - Brett's dog. Brett evidently saddened by his dog's death, throws the car keys to the ground and tells them to leave. Steve and Jenny manage to find the car and start driving away, but get stuck on a steep slope. Meanwhile, Brett has decided he wants retribution after all, and the gang catch up with them and start throwing rocks at the car, breaking the windshield and smashing the headlights. Terrified, Steve and Jenny luckily manage to get the car moving again, but with the headlights out, Steve ends up crashing the car. Finding himself trapped by a tree branch which has come through the windscreen, he tells Jenny to run and to try to find help. Jenny escapes and finds somewhere to hide, where she spends the night.
The next morning she cautiously returns to the car but finds Steve gone. Following a trail of blood, she comes across the gang in a small clearing, with Steve tied up with barbed wire. She watches in horror as the boys - goaded by Brett - take turns torturing Steve with their knives as Paige (Finn Atkins), the girl of the gang, is told to record the whole thing on her phone. Jenny, realizing they are about to kill him uses the bluetooth on her GPS to connect with Steve's phone. Brett notices this, realizing she must be close and orders the gang after her.
Jenny flees, the gang close behind her on their bikes. This gives Steve the opportunity to free himself from the barbed wire, and he also sets off running. Jenny manages to evade the gang, and eventually finds Steve, injured and bleeding heavily. She helps him to a shack where she tries to treat some of his wounds. She finds a wedding ring in Steve's clothing and he tells her he'd planned to propose. The gang turns up and enter the shack but they find nothing as Jenny managed to conceal both herself and Steve underneath the construction in the water. The group leaves, and Jenny pulls Steve out of the water. They decide she should go on alone, leaving Steve concealed under some leaves. Following the power lines Jenny treads on a spike and hides, for fear her screams have alerted the gang to her location. She comes across Adam (James Gandhi), the little boy she and Steve had encountered earlier. She pleads to him for help, but he instead leads her straight to the gang who knock her out.
She awakens to find herself and Steve, who has already died, about to be set alight by the gang. Pouring petrol over them, Brett forces Adam to light the fire with a match. Again Brett commands Paige to film the whole scene with her phone. The rest of the gang are noticeably disturbed at this point. The ropes binding Jenny burn through and she escapes. Brett, in his fury, covers Adam in petrol and threatens to set light to him if she does not return. Jenny though, is already too far away, and can only look on in horror as Adam is burnt alive.
Eventually she finds a trail map, which she removes to help her escape. Narrowly avoiding Brett and Ricky (Tom Gill) by hiding in a large bin, she arms herself with a shard of glass, but before she can go anywhere she encounters another of the gang; Cooper (Thomas Turgoose). Having evidently realized things have gone too far, Cooper seems as if he is about to offer her his help, but before he can do so, Jenny stabs him with the glass. Although the shock of what she has done immediately strikes her, she can do nothing but hold Cooper as he dies in her arms. Finding the body, Paige screams, bringing Brett running. Another member of the gang confronts him, and begins to call someone on his phone, prompting Brett to beat him savagely. Paige then runs off, terrified by Brett's seemingly uncontrollable brutality.
Meanwhile, Jenny finally reaches the road, managing to flag down a van, she gets in, telling the driver she has been attacked. Worried, the driver explains he is looking for his brother, who is also in the forest. His brother 'Ricky'. The driver stops the van and gets out, leaving the keys in the ignition. Jenny, scared that she will be caught again, resolves to steal the van, and speeds off towards town, running over Paige as she does so.
She makes it to town, only to crash into the front garden of a house. Making her way around the back of the house, she finds a party in progress, she pleads for help and collapses. Coming to, she finds herself inside the house, with a woman comforting her. As she looks around and listens to the phone call the parents received from their children, she comes to the realization that she is, in fact, in Brett's house and asks to be shown to the toilet. Brett's father at this point notices the van crashed on the lawn. Finding no escape, Jenny arms herself with a razor. As the commotion builds outside, the door is kicked in and she is confronted by psycho Brett and his psycho father, along with the other party-goers, some of them evidently the parents of the other gang members (including the waitress from the café). Jenny realizes that the gang's parents are just as psychotic and murderous as their kids and they hide and protect them from prosecution. Brett's father orders Brett to his room violently as he and another parent push Jenny back into the bathroom. As the film closes, Brett deletes the videos from Paige's phone and looks into his mirror and makes a very evil smile as Jenny's screams ring from downstairs as she is apparently beaten and killed by his and the other evil parents of the evil gang members.