Two men (Edmond O'Brien and Frank Lovejoy) on a fishing trip pick up a hitchhiker named Emmett Myers (William Talman), who turns out to be a psychopath who has committed multiple murders. Police believe Myers is traveling alone, so he holds the two men hostage to avoid detection as he flees towards the Mexican border and listens to news broadcasts of the police chase on the car radio.
Meanwhile, the men try to plot their escape from the violent, paranoid Myers. They try tactics such as sabotaging their car and leaving clues (like an engraved wedding ring) at various points on their journey. One man badly twists his ankle during an escape attempt. The sadistic Meyers physically and mentally torments the men, forcing them to continue on foot and mocking their loyalty to each other by claiming that they could have escaped separately if they embraced Meyers each-man-for-himself ethos.
Arriving at the a border town, Myers tries to conceal his identity by forcing one of the men to wear his clothes. Myers discovers that the regular ferry to Mexico is broken. He hires a fishing boat but, while waiting for the fisherman, locals discover his status as a wanted murderer and contact authorities. Police surround the pier and, after some confusion over Myers identity, take him into custody following a brief scuffle in which the boastful Myers is revealed as a coward.
The film ends with the weary friends agreeing to give statements to police.