In the opening scene, Jenny Greengrass, a nursery school teacher, and her boyfriend Steve Taylor embark on a romantic weekend getaway to Eden Lake, a remote area in the English countryside. After a long drive, they stop at a motel for the night. The next day, they arrive at Eden Lake and enjoy the beautiful surroundings. 

Their peaceful outing is disrupted by a group of rowdy teenage boys who bully a timid young boy named Adam. Despite the boys' loud music and aggressive behavior, Jenny and Steve try to ignore them. Steve confronts the teens about their dog and noise but is met with disrespect, while Jenny feels uncomfortable when one of the boys leers at her.

That night, while camping, Jenny hears unsettling noises from the woods, but Steve reassures her. The next morning, they discover their food supplies have been infested with insects, prompting them to drive into town for a meal. Unfortunately, they run over a bottle left by the teenagers, causing a flat tire. After Steve fixes it, they visit a diner, where they learn that the waitress doesn't know the troublesome teens. Steve then spots bicycles outside a house and decides to investigate further.