While driving alone in the Mojave Desert, Sandra Thomas's car breaks down after road rage with another driver, later revealed to be a notorious scam artist. She's offered a lift by soft-spoken passerby Jake Nyman, who identifies himself as a Los Angeles psychiatrist on a pleasure trip with decisions based on a flip of a coin. Sandra intended to meet her down-on-luck hitchhiking sister Alice in the town of Pearblossom. However, the rendezvous is once again postponed when Jake's car runs out of gas. During a break at a rest stop, the two grow romantically attached and also reunite with Santini, the temperamental driver who earlier taunted Sandra, and during a drunken row, punched her in the face. Based on a coin flip, Jake offers to either kill Santini or make sure he doesn't hurt her ever again. She's also given the option to accompany Jake for the remainder of his trip using his fate/chance philosophy; However, the outcome of either toss is not revealed, and Sandra and Santini both disappear. Not long after, Jake encounters Alice and also gives her a ride, and while there's common knowledge he was with Sandra, the latter's whereabouts are not known. When a woman's body turns up at a motel she and Jake were staying at, the human remains are identified as town ne'er do well Rita, a flirtatious junkie who slept with Santini. Sheriff Frank and Deputy Sam don't suspect Jake of any wrongdoing based on his clean record and demeanor; the immediate implication is that Santini, a fugitive, was involved in Rita's death and Sandra's disappearance. Alice isn't alarmed that Jake lied to police about why she was with him - claiming that she was a pregnant teen "patient" of his - and continues to goalong for the ride. Later, Santini pursues Jake and Alice on the highway but careens off the road and crashes his car. Alice attempts to rescue him but he's unable to speak due to an injury to the mouth and dies. Sheriff Frank finds a human tongue in the toilet of the motel room Jake stayed in and learns on an APB that he's the suspect in an unfolding serial killer investigation. Concurrently, Jake incriminates himself to Alice as the psychopathic murderer of her sister, coldly reciting their childhood memories and showing Sandra's mutilated corpse in the trunk of his car. Alice hits him in the face with a shovel and escapes to a nearby barn, where Jake ambushes her. As a last ditch effort to save her life and appease Jake's flipism philosophy, Alice proposes a coin toss in which she can either walk free or he'll kill her, using a switched double-headed Kennedy half dollar taken from Santini. Jake discovers the coin is counterfeit and turns his vehicle around to retrieve his coin for a re-toss, but is t-boned by officer Frank and Sam's approaching car, resulting in the deaths of all three.
