Jim Halsey, a young man delivering a car from Chicago to San Diego, spots a man hitchhiking in the West Texas desert and gives him a ride. The hitcher, John Ryder, is brooding and evasive. When Jim passes a stranded car, Ryder forces his leg down on the accelerator. Ryder states he murdered the driver and intends to do the same to Jim, threatening him with a switchblade. Terrified, Jim asks what Ryder wants. He replies, “I want you to stop me.” When Jim realizes that Ryder never put on his seat belt and the car’s passenger door is ajar, he shoves him out the door.
Relieved, Jim continues on his journey. When he sees Ryder in the back of a family car, Jim tries to warn them but becomes involved in an accident. He later comes across the family’s blood-soaked car and vomits. At an abandoned gas station, Ryder corners Jim but simply tosses him the keys he took from Jim’s car. After Ryder leaves with a trucker, Jim encounters him again at another gas station, where the truck nearly runs him down as it crashes into the pumps. As Jim flees, Ryder causes the station to explode.
At a roadside diner, Jim meets Nash, a waitress, and calls the police. He finds a severed finger in his food and realizes Ryder is present. The police arrest Jim, as Ryder has framed Jim for his murders. Though the police doubt his guilt, they lock him up overnight as protocol. When Jim wakes, he finds the cell door unlocked and all the officers dead. He panics and flees with a revolver. At a gas station, he sees two officers, takes them hostage, and speaks to Captain Esteridge, the officer in charge of the manhunt for Jim, on the radio. As Esteridge convinces Jim to surrender, Ryder pulls up and kills the two officers.
The patrol car crashes, and Ryder disappears again. After briefly considering suicide, Jim reaches a cafe, where Ryder confronts him. After pointing out Jim’s revolver is unloaded, Ryder leaves him several bullets and departs. Jim boards a bus, where he meets Nash and attempts to explain his situation. After a police car pulls over the bus, Jim surrenders, and the furious officers accuse him of killing their colleagues and attempt to kill him. Nash appears with Jim’s revolver, disarms the officers, and flees with Jim in their patrol car. As the police chase after them, Ryder joins the chase and murders the officers by causing a massive car accident.
Jim and Nash abandon the patrol car and hike to a motel. While Jim is in the shower, Ryder abducts Nash. Jim searches for her and is discovered by Esteridge, who takes Jim to two trucks with Nash tied between them with a gag in her mouth. Ryder is at the wheel of one truck and threatens to tear Nash apart. Esteridge tells Jim that his men cannot shoot Ryder as his foot will slip off the clutch, which would cause the truck to roll and kill Nash. Jim enters the cab with Ryder, who gives him a revolver and tells him to shoot, but Jim is unable to do so. Ryder, disappointed, releases the clutch, killing Nash. Ryder is taken into custody. Esteridge gives Jim a ride, but Jim, believing the police cannot hold Ryder, takes Esteridge’s revolver and vehicle to chase down Ryder’s prison bus. Ryder kills the deputies and leaps through Jim’s windshield as the bus crashes. Jim slams on his brakes, sending Ryder through the windshield and onto the road. Ryder challenges Jim to run him over, which he does. As Jim leaves his car to observe Ryder’s body, Ryder jumps up, and Jim shoots him repeatedly with a shotgun. Jim leans against Esteridge’s car and begins smoking as the sun sets.