The film begins in the middle of a forest at night. Police officers Morton (Michael Bowen) and Giles (Joshua Cox) are transporting a dangerous criminal, named John Krug (Garret Dillahunt), to prison. Krug requests for them to stop so that he can piss, but they refuse. The car stops at a railroad crossing while a train passes through. Suddenly, a truck crashes into the car. Two masked people get out of the truck and walk over to the car. These people turn out to be Krug's younger brother Francis (Aaron Paul) and Krug's girlfriend Sadie (Riki Lindhome). Giles, extremely bloodied, is still alive and so Sadie shoots him in the head. She then frees Krug and kisses him. Krug asks Francis about his son, and he says that he's okay. Morton is also still alive, though badly injured. Krug strangles him with his seatbelt and they all wonder what Morton's thinking about as he's dying. Francis steals the cash out of Morton's walet, though some blood drips on it. Krug gets Morton's picture of his two little daughters and says that Morton is thinking about what he'll never see again. Krug finishes Morton off.
Elsewhere, 16-year-old Mari Collingwood (Sara Paxton) is swimming in a pool as her mother Emma (Monica Potter) watches. After Mari's swim, she asks her mother what her time was. Emma says that it doesn't matter, and says that they should start their vacation. Emma picks up her husband, Dr. John Collingwood (Tony Goldwyn), at a hospital after he's through with performing a surgery. The Collingwoods drive to the forest to spend their vacation at their summer house. Mari, driving, comes to a fork in the road that has a 'Road Ends In The Lake' sign. She turns left and remarks how ridiculous the sign is. The family arrives at the house and starts to unpack their belongings. Mari chooses to stay in the guest house, which is a little walk away from the house. It turns out that Mari had an older brother, named Ben, who died last year. Its clear that the parents have still not gotten over his death. Mari goes to the boathouse and goes for a quick swim in the lake. She then gets dressed and wears a necklace that Ben gave her. Emma's brother, who recently stayed in the house, has left Emma and John a bottle of wine as a token of appreciation. While John tries to fix the broken microwave, Mari asks her parents if she can borrow the car. She wants to go into the nearby town to visit her friend Paige. Emma thought that they could enjoy a nice family meal together, though John gives Mari the keys and a wad of cash to have fun in town. The only thing Emma asks is for Mari to call her when she gets to town. John and Emma plan on having a nice quiet romantic dinner together.
Mari hangs out with Paige (Martha MacIsaac) as she works at the general store. As they catch up and talk about their plans to get high, a teenager named Justin (Spencer Treat Clark) grabs a bunch of junk food. He approaches the counter and asks for cigarettes. Paige wants to check his ID, and so Justin comes up with a compromise if she lets him buy the cigarettes, he'll give her primo weed that he has in his motel room. Paige winds up agreeing to the deal. Justin gets nervous when he sees that some of the cash has blood on it, but he finds clean bills to pay with. Mari drives them all to the motel. Justin and Paige get out of the car, telling Mari that they'll be back in a couple of minutes. While waiting in the car, Mari sees that her cell phone has no signal. She's startled by a maid, who asks her if she dropped her room key. Tired of waiting, Mari enters the room to find that Justin and Paige are smoking a joint. She's a little miffed that they didn't get her, but she sits down and smokes with them. Emma calls Mari, worried that she didnt call when she got to town. Mari reassures her that everythings fine. As the call starts to break up, Emma warns Mari that a storm is coming. Mari says that shell spend the night at Paiges house and hangs up.
While getting high, Paige dances on the bed. Mari asks Justin about his family. He says that they have to keep on moving due to his father and uncle. He says that his mother is dead. Mari tells him that her brother is dead. Wanting badly to change the subject to something more upbeat, Paige says that Justin could be cute if he didn't have a Unabomber look. She has Justin wear his uncles shirt while she and Mari give him a make-over.
Suddenly, Krug, Francis, and Sadie show up at the motel. Sadie immediately takes off her shirt and walks around topless, looking for a bra and clean shirt to put on. Francis sees that Justin is wearing his shirt, but has him keep it on. Seeing the two teen girls, Krug gets upset with Justin, telling him that he was not supposed to bring anyone to the motel room. Justin tells Krug that he didn't know that they would be back so soon, and says that he thought he could get rid of the weed. Krug wants Justin to stop thinking about his own needs and punches him in the stomach. He tosses a newspaper at Justin, showing him that his escape is all over the news. In the paper are pictures of Krug and Sadie, from a camera inside the cop car. Krug looks at the girls and wonders what to do with them. Mari and Paige promise not to say anything. Francis grabs Paige from behind and pulls out his knife. Mari gives Sadie the keys to her car, hoping that they won't hurt them. Krug apologizes, but he just can't risk having them leave. Paige breaks free from Francis and goes into the bathroom. She tries to use her phone, but it doesn't have a signal. She goes to the window, where she spots a police car in the parking lot. She tries to get their attention, but they don't see her. Francis breaks through the door and throws Paige down from the window, smashing her head against the sink in the process. He then breaks her phone, along with Maris. Krug forces Justin to look at Mari, telling him that hes responsible for their predicament.
The gang and girls drive off in Mari's car. Francis holds onto Paige, Sadie holds onto Mari, and Krug drives with Justin in the front seat. They pass another police car on the road and the girls try to get their attention, but they are both held down. They wind up in the forest and approach the fork in the road. Krug has Francis look at the map to find where the main highway is. Mari tells them that it's on the left, and so Krug turns left. Krug and Sadie compliment Mari on being nice and civil. Mari sees that the car's cigarette lighter is directly in front of her. She pushes the lighter in with her foot while acting calm. She sees that they've passed the road where her house is. When the lighter is ready, she grabs it and burns Sadie's forehead leaving a small burn mark. Paige fights Francis while Krug tries to get everyone to calm down. Mari opens the door and tries to jump out, but Sadie holds her down. Krug winds up losing control of the car, causing them to crash into a couple of trees. Paige, wounded, gets out of the car and tries to free Mari, but Krug knocks her to the ground. Francis finds that his nose broke during the crash. He lets out his anger on the girls, kicking them around. Sadie, pissed that she's going to be scarred for life, beats up Mari. Mari tries to get Justin to help, but he just watches helplessly, too afraid to to anything. Paige and Mari try to escape again by running into the forest. Krug catches Mari before she can get far. He orders Sadie and Francis to get Paige back.
Paige runs through the forest and manages to hide from them. She eventually comes across a construction site and screams for help. They can't hear her due to loud machines at work. She tries to get closer, but is tackled by Sadie. Paige kicks Sadie off her and tries to run again, but is clothes-lined by Francis. He pulls out his gun and is ready to kill her right there, but Sadie says that Krug wanted them to bring her back alive. Sadie rips off Paige's shirt and uses it to tie her hands. They make their way back to Krug, Justin, and Mari. Krug says that its time for Justin to become a man, and asks him which girl he wants. Justin refuses, and so Krug grabs hold of him and forces him to grope Mari. Paige calls Krug pathetic, which surprises him. He walks over to Paige, lifts her up, and stabs her in the stomach with a knife. Francis grabs her from behind and stabs her in the back. Krug then stabs Paige again in the stomach. He lets Mari hold Paige, who struggles to breathe. Krug wants Mari to lie to Paige, telling her that everything's going to be all right. Mari refuses. Krug throws Mari down on the ground. Sadie rips off Mari shorts and helps hold her down. Krug then pulls down Mari's panties and rapes her. Mari screams in pain and sees that Paige is dead. She begs for Justin to help her, but he continues to do nothing. During the rape, Krug rips off Mari necklace and throws it towards Justin (who puts it in his pocket). After Krug finishes, Mari puts her panties back on, degraded. Even the others, for a moment, look a little remorseful about what they've done.
Krug tells Justin that he really missed out on Mari, who says that she has to swim. Mari, having picked up a rock, hits Krug in the head and runs away. Krug tries to shoot her, but misses. Mari gets to the lake and swims away. Krug tries to shoot her, but keeps on missing. Eventually Krug manages to shoot Mari in the back, causing her to stop in the lake and she sinks betheath the water. The storm arrives, and so the gang leaves. Krug has to basically force Justin to walk with them.
While walking through the woods, the four of them wind up on the road leading to the Collingwoods. Emma and John have just finished dinner when they hear banging at the front door. They open to find Krug, Justin, Francis, and Sadie drenched in the rain. Claiming to be a family whose car has broken down, Krug asks the Collingswoods for them. They welcome them inside and John snaps Francis nose back in shape. Emma makes coffee for everyone and hot chocolate for Justin. Krug says that despite their car difficulties, they're lucky to have stumbled upon their house. Krug asks if they can give them a lift to town, but the Collingwoods say that their daughter has the car. Krug asks to use their phone to call a tow truck or taxi, but John doubts that anyone is going to come for them during the storm. While John stitches up Francis nose, the storm causes the power to go out. John goes outside to start the back-up generator for the house, leaving Emma alone with the guests. She asks them what they're doing out there, since she's never seen them before. Francis is ready to use his knife to kill her, but Krug says that their father used to bring them to the lake to fish. He was hoping to spend some quality time with his son Justin (who remains mostly quiet). Francis puts his knife away, and John gets the generator working.
John goes back inside and finishes stitching Francis' nose, during which Francis becomes attracted to Emma. They all go into the living room, where John gets them warm by the fireplace. Justin asks to use the bathroom. Emma tells him where it is, and he goes into the kitchen to put his cup away. He's shocked to see a picture of Mari on the refrigerator. Emma walks in and says that Mari is their daughter. Justin collapses on the floor. He says that he's sick and rushes off to the bathroom. He locks the door and pulls out Mari's necklace. Krug knocks on the door, saying that Emma is worried about him. He tells Justin to get his act together and appear normal in front of the Collingwoods. During this time, we see Mari (badly wounded) making her way to the boathouse and then crawling out of the lake.
John offers his guests to stay there for the night, telling them that they can stay in the guesthouse out back. Justin goes back to the kitchen and places Mari's necklace around his cup. Krug tells him to hurry up, since Emma is going to show them the guesthouse. Since there's no electricity in the guesthouse, Emma lights up a bunch of candles. She says that if they need anything, they can just come over to the main house. Krug thanks her for her hospitality. Emma goes back to the house and doesn't see Mari crawling nearby.
Back inside the main house, Emma says that there's something weird about their guests, but John doesn't think much of it. They hear loud banging outside on the porch. John and Emma go outside to find that the noise is from a rocking chair. They are then shocked to find Mari there.
John sees that shes been shot, and so he carries her into the living room. He heats up a knife in the fireplace and cauterizes the wound to stop the bleeding. He tells Emma to get alcohol and towels from the kitchen. While in the kitchen, Emma finds Mari's necklace wrapped around Justins cup. She realizes who her guests are. Mari is struggling to breathe, and so John turns her on the side and cuts her back. He then places a tube in the cut to help her breathe. John sees blood in her inner thigh and realizes that shes been raped. Emma tells John that their guests did this to their daughter. She saw Mari wearing the necklace before she left, and she knows Justin left it around his cup. John grows furious, and Emma asks him what theyre going to do. The nearest house is miles away, they don't have a car, and the gang is still there. John tells her that they need to be prepared to do anything. Since Mari needs a hospital, they plan on using their boat to travel there.
Meanwhile, in the guesthouse, Sadie takes a shower and Krug gets ready for bed. He sees Justin sleeping on the couch in the other room. It turns out Justin is pretending to be asleep, and he starts to eye Krug's pistol. Sadie comes to bed topless and she falls asleep with Krug. With Francis asleep in a chair nearby, Justin sneaks into the bedroom and steals Krug's gun.
John goes to the boathouse and gets the boat ready, but finds that the keys are missing. While looking around, he starts to eye some tools nearby. He grabs a hammer to use as a weapon to kill the guests. Emma eyes some knives in the kitchen, and then Francis suddenly shows up. He says that the lightning outside woke him up, and he was wondering if he could have something to drink. To keep him from seeing Mari's picture on the refrigerator, Emma flirts with him and says that John is passed-out upstairs. She offers some wine to Francis, and tells him to wait in the living room while she gets glasses. Francis walks into the other room and sees Mari curled up in front of the fireplace. He recognizes her and is about to panic when Emma smashes a wine bottle over his head. Enraged, he charges at her, but Emma stabs him in the chest with a butcher knife. Francis beats her up a bit and goes over to the window in the dinning room to try to warn the others in the guesthouse (but they can't hear due to the storm). John shows up and beats Francis with the hammer. He also re-breaks Francis nose. While fighting, they wind up back in the kitchen. Emma and John hold Francis head down in the sink, drowning him. While thrashing around, Francis' hand winds up in the garbage disposal. John turns it on, slicing up Francis' hand. As Francis screams in pain and tries to break free, John stabs him in the head with the hammerclaw, killing him.
John and Emma sneak into the guesthouse, armed with the butcher knife and a fire poker to kill the others. They approach the bedroom and see Krug and Sadie in bed, but then see Justin sitting nearby. Justin aims the pistol at them, but then offers it to John. As quietly as he can, John slowly walks across the bedroom and is handed the pistol. When he cocks it, Sadie wakes up. John shoots at her gazing her neck, but also waking up Krug. Sadie jumps on John and fights with him for the gun as Krug throws the bed at Emma. John throws Sadie in the bathroom and then tries to shoot Krug, but he jumps out of the window and makes his way towards the house. Sadie grabs a pole from the shower and uses it to beat John up, knocking the gun out of his hands. Justin tries to help John, but is knocked out by Sadie. Emma grabs the pistol and shoots Sadie through her left eye, killing her.
Krug gets to the house, somewhat confused as to why the Collingwoods are attacking them. He then looks in the living room and finds blood on the table. He walks into the kitchen and finds Francis' body. Krug gets really angry and storms off. It turns out that the Collingwoods put Mari in the boat before they went to the guesthouse. Emma stays with her while John goes to the house with the fire poker to kill Krug. John finds that his wallet has been placed on the table in front of the fireplace. Krug has seen his family photo, and knows that Mari is their daughter.
Upstairs, Krug taunts John to come up and get him. Krug rolls an object on the floor and keeps moving around to throw John off. He looks outside and realizes that they're going to use the boat to escape. He opens a window and makes a loud noise, causing John to rush upstairs. Thinking Krug has escaped through the window, he tries to yell out to Emma. It turns out Krug was hiding in the room. He ambushes John, and they beat the crap out of each other. Krug kicks John through the railing on the stairs, causing him to fall to the ground floor. Krug then stands over the wounded John and asks him if he wants to know how tight his daughter was. John says that he wants to hear him beg for his life and breaks a table over Krug. After more fighting, Krug chokes John with the fire poker until he passes out. John falls to the floor and Krug prepares to stab him when he hears a gun cocked behind him. Krug turns around to find Justin aiming the gun at him. Krug can't believe it, telling him he picked a hell of a time to man up. Krug says that he's not angry, and asks for the gun. Justin pulls the trigger, but the gun is out of bullets. Enraged, Krug stabs Justin in the stomach with the fire poker. Emma comes back with a fire extinguisher and hits Krug with it. She sprays him with the hose and John beats him with the fire poker. Emma then knocks Krug out by bashing his head with the fire extinguisher. The storm clears up outside. John, Emma, Mari, and Justin all get in the boat and drive down the lake.
Some time later, Krug wakes up alone to find John standing over him. Krug says that he can't move. John says that he didn't have any rope or tape to hold him, and so he made cuts all over his body to paralyze him from the neck down. Krug, helpless, asks him what he's doing. John puts Krug's head in the broken microwave and turns it on. The film ends with Krug's head exploding in the microwave.