David Mann (Dennis Weaver) sets out from Los Angeles on a business trip driving a 1970 red Plymouth Valiant. His eastward path takes him from the crowded freeways to a long stretch of 2-lane interstate crossing the California high desert. Mann's journey is going reasonably well until he encounters a slow-moving tanker truck. The truck is grimy and oil-stained, and constantly belching black smoke. Mann passes the slow-moving truck, but soon after, the truck roars past him, forcing him to steer sharply to avoid a collision, then returns to its slow-moving speed. Mann rolls his eyes and passes the truck a second time, eliciting a loud, startling blast on the truck's air horn.Mann soon pulls into a small gas station, into which the truck follows him. The driver of the truck, unseen except for a pair of hairy bare arms and blue jeans with brown cowboy boots, is watching Mann through his rear view mirror. He instead exits the truck on the passenger side and walks alongside it, not wanting Mann to see his face. Mann goes inside the station to phone his wife (Jacqueline Scott) at home who is a bit annoyed with him for not confronting one of their friends at a recent party who was making a pass at her. Mann argues that the friend had too much to drink and did not require that kind of overreaction. His wife's anger cools but she chides him for always being one who tends to avoid confrontation, leaving him feeling weak and emasculated. Afterward, the gas station attendant fills up Mann's car with gas and also mentions that Mann needs a new radiator hose for his engine, but he casually puts that off for the moment.Mann returns to his car and continues his drive. He assumes it'll be smooth sailing from here on, but soon the truck approaches him from behind, quickly. Mann politely gestures to the driver to go ahead and pass him, which he does, but then slows to a ridiculously low speed. Highly annoyed, Mann complains out loud, as if to the driver, eventually blowing his horn. Shortly thereafter they reach a passing lane where Mann tries to drive past the truck, but each time he does, it swerves back and forth, blocking his attempts. Mann grows more and more annoyed and for the first time realizes that what might earlier have been seen as simply extreme discourtesy is really a deliberate attempt to frustrate and harass him.The trucker's left arm emerges from the cab window, and encourages him to pass. "Well it's about time, Charlie!" says Mann, before suddenly seeing an oncoming vehicle heading towards him! Mann quickly swerves back into the right lane, stunned at the realization that the driver intended to kill him!Through the rest of the mountainous climb to the high desert, a very shaken Mann continues to try to pass the truck and grows more and more frustrated as he is blocked every time. The game continues as the slopes give way to the flat, arid terrain of the desert, until he sees a turnout on the opposite side of the road. He accelerates and races through it, passing the truck and giving the driver a gleeful snub, assuming he'll now be pulling far ahead of the truck and the incident will be behind him.Before long, however, he is shocked to see the truck pull up behind him at high speed. Mann attempts to outrun the truck and pull ahead, but the truck speeds up as well. Mann is soon freaked out even more when the truck pulls up on him, reaching speeds above 90mph. The truck bumps into Mann's vehicle several times, until they reach a diner (Chuck's Cafe), where he swerves off the road at high speed, coming to rest after striking and damaging a wooden fence. A few friendly locals sympathetically try to help, but there's nothing much they can do.Mann enters the diner to compose himself. After returning from the restroom, he is shocked to see the truck parked outside the diner. Mann studies the diner patrons carefully and begins an inner monologue in which he contemplates the driver's motives as well as the truck's remarkable performance, and second-guesses his decision to sit helplessly in the diner. Most of the patrons sitting at the counter give Mann the impression of malice, but when one leaves, appearing to approach the tanker, he instead drives away in a pickup truck.Mann continues to look around the diner, and soon believes he's found the culprit sitting alone at a table. Approaching the stranger, Mann tells the man to "cut it out". The stranger looks at Mann as if he's crazy, and when Mann continues to accuse him, a fistfight breaks out. When the manager of the diner demands the fight stop, the stranger leaves.... only for Mann to see him get into a different truck. Mann leaves the diner, and the dirty semi roars to life; apparently the driver never entered the diner. As it takes off, Mann runs after it a ways, but as it pulls away, he stops, and watches it recede into the distance.Mann gets back in his car and continues on his way, soon encountering a stalled school bus. The driver asks Mann to give him a push, but Mann's small red Valiant is hardly equal to the task and ends up getting its bumper hooked under the rear of the bus. As he gets out to tell the bus driver, he panics: a short way further on, up in a darkened tunnel, he sees the semi is back, and approaching! Frantically, he manages to free his vehicle and flee, just as the semi truck arrives and pushes the bus (though the driver is still not seen).Mann continues on his way, stopping at a railroad crossing. The sound of the train is so loud that Mann doesn't hear the truck creeping up on him. When it begins to push Mann into the side of the still-moving train cars, Mann attempts to reverse his car to keep from going further. (Note: although the truck could easily overcome any resistance the car might offer, it doesn't, showing that the driver intends to torment him but not injure him... yet.) Just as it seems his efforts have failed, the train finishes going past, and Mann throws the car into 'drive.' Mann drives a ways on, before he veers off the road, with the truck continuing on.A short distance ahead, Mann pulls into a place called Sally's Snakerama Gas Station to call the police and refuel his Plymouth while the truck is seen pulling to a stop ahead on the road. He goes into a phone booth to call the police, but before he can get through, the truck starts up, turns around, roars up, and plows into the telephone booth, with Mann barely jumping clear in time. The truck proceeds to chase Mann as he is on foot, destroying the Snakerama. Terrified, Mann jumps into his car and speeds away. He then hides behind an embankment off the road and sees the truck pass by, apparently without noticing him. He thinks to himself that he'll just wait "at least an hour," sure the truck will be long gone, and dozes off in exhaustion. Shortly thereafter the train approaches, and in his tortured sleep he confuses the sound of the locomotive's horn with that of the truck, until the train bursts past him and he awakens fully, suddenly laughing almost hysterically in relief.Mann then heads off again but is dumbfounded to see that the truck is waiting for him a few miles ahead, just around the bend! Mann stops his car, gets out, and attempts to walk up to the truck to confront the driver and ask why he is chasing him. But when Mann approaches to within a few yards from the truck, the driver (still unseen except for a silhouette) starts up the truck and drives off up the road a short distance, taunting him. The driver clearly does not want to establish any visual or verbal contact with Mann... or let him see his face.Mann returns to his car and attempts to get help from an older couple in a car that is passing by. But the elderly and senile couple think that he is crazy due to his frantic and desperate tone of voice... until they see the truck themselves, backing up towards them at increasing speed. After the elderly couple drives away leaving Mann behind, the truck changes direction and approaches Mann's car, seemingly intent on wrecking it. Mann flees off the road on foot into the desert, but just then the truck stops short, then moves ahead again. By now Mann's state of mind has progressed from victim to warrior; the real duel has begun. Aware that he is on his own, Mann returns to his car. The truck driver, being careful not to let Mann (or the viewers) see his face, sticks his left arm out of the truck's driver's side window and again beckons Mann to pass him. Mann starts his car, shifts into drive, passes the truck, and a climatic high-speed chase begins.Mann again finds himself unable to outdistance the truck, but takes comfort knowing he'll soon reach a steep grade of a nearby mountain on which the truck can't possibly keep up. When he reaches the grade for a steep hill, he is able to put some distance between himself and the truck and starts to relax. But just as he calms down and regains his confidence that he is out of danger, the weak radiator hose fails and the now-overheated engine can no longer deliver more than barely enough power to take the car to the summit. The truck quickly begins gaining on him. Mann barely makes the summit and coasts down the other side as he shifts his car into neutral just as the truck bears down on him.Mann allows his car to coast down the hill, giving the engine a chance to cool down. Descending at speeds too great to control, the red Plymouth Valiant spins out and impacts against a rock wall. The truck speeds toward the damaged car as Mann desperately tries to get it started, succeeding just in time to avoid the truck. He drives off the road and up a dirt road, reaching a dead end which is a locked gate to a nearby ranch that sits beside the edge of a massive cliff. Mann turns his car around to face the oncoming truck. Wedging his briefcase on the accelerator, Mann sets his car right into the path of the truck, jumping out at the last second. The driver of the truck hits the car, but upon realizing there's nowhere left to go, he jams on the brake, but too late. Mann's car and the truck, with the driver still inside, plummets over the side of the cliff wall, collapsing into a pile of twisted metal wreckage and dust at the bottom.Surveying the smoking wreckage of the truck and his car at the bottom of the cliff, the clearly delirious Mann runs around the top of the cliff giggling and jumping (resembling some Neanderthal cave man after having slain a large cave bear) as he celebrates his victory and then sits down, exhausted, on the edge of the cliff. He tosses stones over the cliff as the sun sets and the closing credits roll in silence.