Jim (Zachary Knighton) is a young college student picking up his girlfriend Grace (Sophia Bush) to go to Lake Havasu for a Spring Break holiday. They zoom along the highway in Jim's muscle car, an Oldsmobile 442, and by evening they are in New Mexico and in a rainstorm. Jim frequently gives Grace loving glances, and as he talks to Grace about meeting her friends, Jim looks away from the road for a little too long. He almost hits a man standing in the middle of the highway as the rain pours down. Jim stops the car, feels bad that he almost hit this guy, and wants to offer the man a ride as it seems the stranger's car parked by the side of the road has broken down. Grace says absolutely not to pick him up, but the man now is walking towards their car. Jim tries to start his car but the motor is flooded. It finally starts and they drive off, just as the man reaches their car door.
They argue about whether or not they should have picked up the man; Jim feels guilty. A few miles down the road they stop at a gas station with the typically weird attendant. Jim asks the attendant to send a tow rig back to pick up the stranger and his car, but the attendant says that will have to wait until the next day. Grace goes to the lady's room as Jim buys her favorite "processed food" - Hostess Twinkies. As he fills his car with gas, a big rig stops and drops off a passenger who is the stranger they left stranded. The stranger asks the attendant where the nearest motel is located, and when he hears it is 12 miles down the road he turns to Jim and asks if he can ride with him. They talk about the fact Jim didn't stop for him earlier, and the stranger says he doesn't blame him. He wouldn't have stopped either. He says his name is John Rider (Sean Bean), and Jim agrees to take him as far as the motel just as Grace comes out of the lady's room looking very apprehensive. For the first minute of their ride down the highway, John Rider seems friendly and pleasant. The two men are sitting in the front and Grace is in the back listening to her iPod. Then John Rider casually asks Jim how often he has sexual relations with his girlfriend. Jim is stunned and upset. He notices the ring on John's left hand and asks him how often he has sexual relations with his wife. John says he's not married and that he wears the ring so that people will think he's a nice guy. John then reaches for Jim's cell phone and snaps it in two. As Grace tries to dial 911 on her phone, John Rider pulls out his knife and holds it to Jim's throat and says he will kill him unless he says, "I want to die." Jim continues to drive and he and Grace are in a panic as he slowly says, "I want to....." As he says "die", he swerves the car, Grace opens the door and John Rider falls out onto the road.
Jim and Grace continue driving, badly shaken, agreeing to tell the police what has happened at the next town. A few hours later, Grace is asleep in the car and Jim is so tired he pulls off the side of the road to take a nap. He falls asleep, but John Rider finds them and smashes in his car window..... Jim wakes up realizing he was having a nightmare. He and Grace continue on down the highway and are passed by a station wagon filled with a dad, mom and two kids in the back. Sitting next to a little boy in the back end of the station wagon is someone holding a giant stuffed frog in his lap. As the station wagon passes Jim and Grace, the frog waves at them, and we see the smiling, friendly face of John Rider pop out from behind the frog. Jim and Grace freak out as they realize what John Rider will do to this family. They speed up and come even with the station wagon as Grace screams out a warning to the man and woman. They don't get what she is trying to tell them, and Jim loses control of his car, and drives off the side of the road and into a ravine.
The car is totaled, but Jim and Grace manage to pull themselves together and climb back onto the road and continue their journey on foot. Soon they see the station wagon pulled off the side of the road, and they run to it fearing that John Rider has used his knife to kill the family - which proves to be the case. Jim holds the dad, the only one still alive as Grace drives. Suddenly a red pickup is banging into the back of the station wagon. It's John Rider trying to run them off the road. Grace drives to the next stop on the road, a cafe. The truck has dropped out of sight. She runs inside and tells the waitress to call 911 and asks for towels (to help stop the father's bleeding wounds). The waitress calls the sheriff as Grace heads for the bathroom and begins pulling paper towels from the holder. Suddenly she sees that someone is outside the bathroom door (shadows under the door threshold) and she ever so quietly slides the door lock closed. The door knob turns. Grace waits until the person leaves and she thinks it's safe and runs back into the cafe where she is arrested. The sheriff and his deputies arrive on the scene and think Grace and Jim are the killers since they are strangers in their community, and take them both into custody. They lock Jim in a jail cell, and ask Grace questions in an interrogation room with a see-through mirror. The sheriff steps out of the room, and doesn't return. She looks at the mirror and realizes someone is looking at her from the other side. That someone is John Rider who has killed the sheriff and his two deputies. He dips his finger in the sheriff's blood and traces Grace's face as she peers through the other side of the mirror/window.
Grace finally tries the door of the interrogation room, and when it opens, she grabs the keys off the sheriff's body and unlocks Jim's cell. As they run out the back door of the jail, they take a gun off the body of one of the deputies. John Rider has disappeared for the time being. The state police now are involved and as the lieutenant in charge walks through the carnage in the sheriff's office, he sees the drawing in blood on the interrogation room window and realizes that Jim and Grace are not the only killers. When he gets their background information and sees that they are college students, he says they may be innocent.
Jim and Grace make their way to an old trailer park and try to find a car. When a deputy arrives, they hide inside of an old building, and a scorpion crawls near Jim's foot. Instead of stepping on it, he moves it away with his foot. When the deputy tries to arrest them, they manage to get his gun. As they try to get him back into his car, he tries to get the gun back from Grace. A shot rings out, and there is a bullet hole in the deputy's head. Another carload of lawmen drive up and think Grace shot the deputy, but it was John Rider who has been hiding in a building just behind Grace, and who used a rifle he took from the sheriff's office.
Jim and Grace drive off in the dead deputy's car with three police cars and a helicopter right behind and over them. Grace speaks into the police car radio microphone and says they are innocent. But the overzealous lieutenant who is monitoring all of this from his office says that his men saw her shoot the deputy. A policeman shoots the tires on their car and the helicopter hovers right above them. They must pull over, but, almost out of nowhere, a black and white Trans Am pulls up behind the three police cars, and then alongside them. It's John Rider and he shoots the officers and when one car crashes into another and another, and they are all killed. He looks up at the helicopter and shoots the pilot and it crashes.
Jim and Grace manage to drive their thrashed sheriff's car to a motel and sneak into one of the rooms. They shower together, and Jim says he's sorry for getting them into this mess. He tells Grace he is going a pay phone to call home to get help and will be back in 15 minutes. Grace finishes her shower and watches TV in bed and falls asleep. When she awakens, she sees a Hostess Twinkie on the night stand and smiles. She thinks Jim is in bed with her, but it turns out to be John Rider. She struggles with him, smashes his face with a lamp, and grabs the gun she has been carrying with her. She hides in the bathroom and gets dressed. She comes out of the hotel room, carrying the gun, calling for Jim. She finds him with his arms chained to the front of one big rig and his legs to the back of another. She runs to the driver's cab of the first truck and there sits John Rider ready to drive the truck forward which will kill Jim. He tells Grace to shoot him - that he wants to die, but that if he does, his foot will come off the clutch and Jim will die anyway. Grace can't pull the trigger, and the police show up just in time to watch the truck pull forward, where they seen Jim ripped in half (you see Jim ripped in half on-screen) and arrest John Rider.
Next we see Grace, in shock, sitting in the lieutenant's office. The lieutenant says he will take her to Albuquerque where they have a trauma facility and that her parents will meet her there. He also spends a few minutes interrogating John Rider.... or the stranger who has assumed his identity. Apparently, the real John Rider disappeared the previous Tuesday, and they cannot get any kind of identification on this killer of so many people. The policemen escort the killer to a van to be moved to Albuquerque and begin the journey with the lieutenant and Grace following in the lieutenant's car. We see the interior of the back of the van where the killer sits with a single police officer. He works the handcuffs off his wrists, slashes the neck of the officer with the loose handcuff, grabs the officer's gun, and shoots the officers in front as the van crashes into the lieutenant's car. The lieutenant's leg is pinned in his car and he can't move. Grace takes his gun determined to stop the killer once and for all. She moves toward the van and opens the back door. The killer leaps out, grabs the gun from her, locks her in the back of the van, and then sets the van on fire.
The killer walks toward the lieutenant, carrying his gun. Meanwhile, Grace is trying to squeeze through the grate separating the back of the van from the front. Finally she gets through far enough to grab a shotgun the guards carried with them. She shoots out the back of the van just as the killer shoots the lieutenant. She walks toward the killer, cocks the shotgun and shoots him in his bulletproof vest. He turns to walk away, and she shoots him again. He kneels in front of her and says, "It feels good, doesn't it." And then she shoots him in the head as the camera pans away showing Grace standing with the shotgun, and the killer lying in a pool of his own blood.