Primary school teacher Jenny (Kelly Reilly) and her boyfriend Steve (Michael Fassbender) head out of London for a weekend break at a beautiful secluded lake. Steve has chosen the romantic spot, and is waiting for the perfect moment to surprise Jenny with a marriage proposal.
They pitch a tent, go swimming, sunbathe, and enjoy the peace – until it is shattered by a group of rowdy, threatening teenagers. When Steve confronts the group about their behaviour, a fight breaks out, and Steve accidentally kills gang leader Brett’s dog.
Suddenly the couple’s idyllic getaway turns into a survival nightmare, as they are forced into the forest to escape the vengeance of their pursuers. Separated from Steve, Jenny has to draw on unknown reserves of strength– and mercilessness – to survive.
Finally breaking free from the forest and her young pursuers Jenny makes it to the apparent sanctuary of the town. And meets the parents.