Jim Halsey, a young man delivering a car from Chicago to San Diego, spots a man hitchhiking and gives him a ride. The man, John Ryder, is a brooding, soft spoken man; when Jim passes a stranded car, however, Ryder's personality suddenly shifts. Ryder calmly states that the reason the car is stranded is because he murdered and mutilated the driver, and he intends to do the same to Jim. Ryder produces a switchblade knife and taunts Jim for several moments, before Jim realizes Ryder never put on his seat belt and knocks him out of the ajar passenger door.
Relieved Jim continues on his journey, until a station wagon carrying a family on vacation passes, and Jim is horrified to see that Ryder has hitched a ride with them. Jim attempts to signal the family, but ends up nearly totaling his car; when he finally catches up to the station wagon, Jim discovers the entire family has been hacked to death.
The movie shifts into an elongated chase sequence, which finds Jim trying to flee Ryder both on foot and in his car, to no avail; wherever Jim runs, Ryder finds him: It seems that Ryder, impressed with Jim's show of bravery by knocking him out of the car, has decided that Jim is the man who is finally going to put his killing spree to an end.