David Mann, portrayed by Dennis Weaver, embarks on a business trip from Los Angeles, cruising in his striking red 1970 Plymouth Valiant. As he ventures eastward, he finds himself navigating away from the bustling freeways into a desolate stretch of two-lane interstate that meanders through the California high desert. Initially, the journey seems smooth, until he encounters a filthy, slow-moving tanker truck spewing black smoke. With irritation mounting, Mann attempts to overtake the truck, only to be met with a menacing blast from the truck's air horn as the driver suddenly accelerates, forcing Mann to swerve sharply.
Stopping at a small gas station, Mann becomes aware of an unseen driver watching him closely, revealed only by a pair of hairy arms and worn jeans. He tries to connect with his wife, Jacqueline Scott, over the phone, where they clash over his avoidance of confrontation, leaving him feeling vulnerable. The gas station attendant points out that Mann's car could use a new radiator hose, but he brushes it off, thinking it can wait.
Upon resuming his drive, Mann politely waves the truck driver ahead, but the truck retaliates by reducing speed, engaging Mann in a twisted game of cat and mouse. Each attempt he makes to pass is thwarted by the vicious maneuvers of the truck, revealing a sinister intent behind the wheel. After a near-death experience dodging an oncoming vehicle, Mann realizes that the truck’s driver is actively trying to harm him.
As Mann presses on, frustration escalates, leading to encounters at a diner where he suspects the identity of his tormentor among the patrons. A confrontation ensues, resulting in a fruitless fight that leaves Mann bewildered when the supposed aggressor drives away in a different truck, while the menacing tanker remains parked outside, unchanged.
The tension mounts as Mann encounters obstacles like a stalled school bus and a perilous train crossing, where the relentless truck pushes him to the brink. In a frenzied attempt to call for help, his phone booth becomes a target, but he narrowly escapes being crushed. Seeking refuge, he hides only to be stunned by the truck's persistence, stalking him without relent.
In a dramatic twist, Mann's desperate fight for survival morphs into a fierce clash between the two. After a high-speed chase through treacherous terrain, his car's radiator hose fails, leaving him vulnerable as he careens toward a rocky precipice.
In a final showdown, Mann fortifies himself against the truck by wedging his briefcase on the accelerator before making a narrow escape just as the truck plummets off the cliff, crashing into a mangled wreck below. With the chaos settled, a giddy Mann, overwhelmed by adrenaline, engages in a primal celebration atop the cliff, throwing stones into the abyss as the sun sets, marking the end of his harrowing battle against the unseen menace of the road.
