After settling their affairs, Emma (Monica Potter) and John Collingwood (Tony Goldwyn), and their daughter, Mari (Sara Paxton), head out on vacation to their lake house. Shortly after arriving, Mari borrows the family car and drives into town to spend time with her friend Paige (Martha MacIsaac). While Paige works the cash register at a local store, she and Mari meet Justin (Spencer Treat Clark), a teenager passing through the town who invites them both back to his hotel room to smoke some marijuana. While the three are hanging out in the hotel room, Justin's family members return: Krug (Garret Dillahunt), Justin’s father; Francis (Aaron Paul), Justin’s uncle; and Sadie (Riki Lindhome), Krug’s girlfriend.

Krug shows Justin a local newspaper that has Krug’s and Sadie’s pictures on the front page, and which explains how Sadie and Francis broke Krug out of police custody and killed the two officers that were transporting him. Believing it too risky to let Paige and Mari go, the gang kidnaps them and uses their vehicle to leave town. While Krug searches for the highway, Mari convinces him to take a road that leads him to her parent’s lake house; Mari then attempts to jump out of the vehicle, but the ensuing fight amongst all the passengers causes Krug to crash into a tree. Frustrated by Mari’s attempts to escape, Sadie and Francis proceed to beat Mari as she crawls away from the wreckage. Krug attempts to teach his son to “be a man” by forcing him to fondle Mari’s breasts. When Justin refuses Krug decides to rape Mari. Paige begins insulting him to get him to stop, which causes Krug to pause momentarily only to stab Paige with his knife. As Mari watches Paige bleed to death she is raped by Krug. Afterward, Mari musters enough strength to escape the group and make it to the lake so that she can swim to safety. Before she can swim far enough Krug shoots her in the back, leaving her to bleed out in the lake.

A storm forces Krug and his gang to seek refuge with John and Emma, whose home is nearby. Justin realizes that they are Mari’s parents, and intentionally leaves a necklace Mari was wearing on the counter to alert them about their daughter. When John and Emma find Mari barely alive on their porch, and the necklace on the counter, they realize that the people who did this to Mari are in their home. As they try and find the key to their boat, so that they can take Mari to the hospital, they decide to get revenge on those responsible. When Francis happens upon Mari he is attacked and killed by John and Emma. When the couple goes after Krug and Sadie they find Justin in possession of Krug’s gun; Justin gives the gun to John so he can kill Krug. Sadie awakes and interrupts John, allowing Krug to escape the couple: he then realizes that they are Mari’s parents. After Emma shoots Sadie in the head, John chases Krug. With a combined effort from Emma, John, and Justin, Krug is knocked unconscious. John paralyzes Krug from the neck down, and leaves him to die with his head in an active microwave oven; then he, Emma, Mari, and Justin take the boat to the local hospital.