Nan Adams, a 27-year-old department store buyer, is traveling across country from Manhattan to Los Angeles. En route Nan has a blowout in Pennsylvania. A mechanic arrives to help her out and tells her to follow him into town so that he can replace her tire. As Nan starts up her car, she sees a short drab man try to hitch a ride. Nan ignores him and follows the mechanic into town. Once he replaces the tire, Nan pays him... and sees the hitch-hiker in her cosmetic mirror as she checks her cosmetic case. Nan mentions seeing the hitch-hiker, but the man is gone. The mechanic wishes her a safe trip and goes inside. On the road, the hitch-hiker watches Nan pull away.
As she continues along the highway, Nan sees the hitch-hiker 50 miles along. She sees him again miles later. No matter how far she goes, Nan still sees the man ahead of her. She's afraid, but doesn't know why. Nan stops at a diner for food and asks about hitch-hikers. He says that they're rare on the turnpike because no one would stop for one on the lonely stretch of high-speed road. Nan says that she didn't see any hitch-hikers and starts to go, but stops long enough to stay that she's starting to hate her car.
Nan drives and comes to a construction site. As she parks and starts to get out, the hitch-hiker comes up and asks if she's heading west. Nan all but screams that she isn't, and then drives off through the construction.
Later, Nan comes to a railroad crossing and waits. She sees the hitch-hiker ahead of her and drives over the crossing... and her engine stalls. The train approaches and Nan tries to start her car as the hitch-hiker calmly watches and beckons to her. With seconds to spare, Nan gets her car moving off the tracks. When the train passes, the hitch-hiker is gone. She figures that the hitch-hiker wanted her to die, and that he'll always be ahead of her.
For three days Nan continues driving and the hitch-hiker is always ahead of her. On the fourth day in New Mexico, she takes a side road in the hopes of escaping her pursuer. Nan runs out of gas and nervously starts walking to the next gas station. It's closed and Nan wakes the owner, John Thompson. He tells her to come back in the morning, but Nan says that she can't stay out alone. She explains about the suspicious-looking man, but admits that the hitch-hiker hasn't done anything to her. The owner closes his window, and a sailor taps Nan on the shoulder. He explains that he's heading back to his ship in San Diego, and Nan offers him a ride. The sailor eagerly agrees and wakes up the owner.
Once they have the gas, Nan and the sailor go back to her car. They're soon on the road and the sailor says that she looks like a movie star. He figures that no one will believe a beautiful woman picked him up, and Nan makes small talk. She begins to get nervous and speculates about how someone could pick up a hitch-hiker and keep ahead of somebody in a slower car. Nan spots the hitch-hiker up ahead and swerves, and the sailor grabs the wheel. He says that he didn't see anyone, but Nan insists that she did and tries to swerve. The sailor fights for the wheel and they pull over, and he offers to drive. Nan admits that she was trying to run over the hitch-hiker and stop him, and the sailor says that he's getting out. She asks him to stay, saying that she likes him, and hopes that he'll take her out. The sailor considers but then gets out, ignoring her pleas. Before he goes, he tells Nan to get a good night's sleep.
Later, Nan pulls into a diner parking lot and uses the pay phone to call her mother so she can hear a reassuring voice and hold onto her sanity. A neighbor, Mrs. Whitney, says that it is the Adams home. She says that Nan's mother is in the hospital with a nervous breakdown... since the death of her daughter. Mrs. Whitney explains that Nan was killed six days ago in a car accident in Pennsylvania when her tire blew out.
Shocked, Nan drops the phone and leaves the booth. All of her emotions fade away, and Nan figures that the hitch-hiker is out there somewhere waiting for her... and she'll find out what he wants. However, she thinks that she knows who he is. Nan gets into the car and sees the hitch-hiker in her rear seat. He figures that she's going his way, and Nan sits alone.