The driver and mechanic, known only as "the Driver" and "the Mechanic," respectively, are expert car builders and drivers. They take pride in the fact that their car, a 1955 Chevy Bel-Air, has been completely stripped down and rebuilt for the sole purpose of racing. They travel around the country looking for challenges and betting money on their races.
Their routine is interrupted when they discover a young hitchhiker, a girl who calls herself "the Girl." She talks nearly incessantly, but neither the Driver nor the Mechanic seem to mind her presence. In fact, they soon become protective of her when they realize that she is running away from something.
At the same time, the trio finds themselves facing off against a well-to-do drifter who is driving a brand new Pontiac GTO. This older man, known only as "GTO," is fascinated by the Driver and the Mechanic's car and begins to challenge them to races. The Driver and the Mechanic are initially dismissive of GTO, but as the races become more heated, they begin to take him more seriously.
As they make their way from one drag strip to the next, the relationships between the characters begin to shift. The Driver and the Mechanic become more protective of the Girl, who they suspect may be in danger. GTO becomes more and more antagonistic, desperate for attention and recognition.
In the end, the group arrives at a massive, unofficial drag race in Washington, D.C. The tension between GTO and the other characters reaches a boiling point, and a final race is arranged to determine once and for all who is the better driver. The race is intense and ultimately tragic, as the characters' personal demons come to the fore.
Although Two-Lane Blacktop is primarily a movie about cars and racing, it is also a meditation on American culture in the 1970s. The characters are all classic archetypes, representing different aspects of American society: the loner, the mechanic, the drifter, the girl. The movie also explores themes of alienation, disillusionment, and the search for identity.