It is a thriller about a traveling salesman, played by Dennis Weaver, driving home on the back roads of the California desert.
Inexplicably, he is terrorized by a large truck, which repeatedly chases and attempts to run him off the road. The driver of the truck is never seen throughout the movie (except in one shot where his arm beckons Weaver to pass him), and his motives for targeting Weaver's character are never revealed.
The story struck a chord with early 1970s audiences. At this time, the Vietnam War was in full swing, and the nation had recently been rocked by the social unrest of the civil rights movement and the assassinations of such prominent figures as John F. Kennedy, Robert F. Kennedy and Martin Luther King. The disastrous Rolling Stones free concert at Altamont made the youth counterculture seem frightening and dangerous. All in all, public perceptions at this time were that the world was a chaotic and unpredictably violent place.
Despite its simple plot and low budget (only $375,000), the movie maintains a high level of suspense due to Spielberg's taut direction and the script's refusal to resolve the central mystery of the driver. The film's success put the young Spielberg on the map in Hollywood, and enabled him to move beyond directing for television.
There is debate among fans of the film over whether the truck is driven by a lunatic, or whether it is actually a 'ghost truck'. The latter seems improbable, as other characters in the film witness the truck also.
The script is adapted by Richard Matheson from his own short story. It may have been inspired by a 1947 episode of the old-time radio series Lights Out entitled "What the Devil", which had a similar plot but different resolution.