Frenchmen Mario and Jo, alongside Bimba from Germany and Luigi from Italy, find themselves trapped in the desolate town of Las Piedras. This isolated place is encircled by harsh desert, and its only connection to the outside world is a dilapidated airstrip, far too expensive for the men to utilize. With minimal job opportunities, they have become reliant on the tyrannical Southern Oil Company (SOC), which dominates the town and exploits the local workforce while ensuring its control with little regard for the law.
Mario, a sarcastic Corsican playboy, takes his loving partner Linda for granted, while Jo, an aging ex-gangster, struggles with the reality of his situation after becoming stranded. Bimba, harboring deep-seated anger due to the murder of his father by the Nazis, is a reserved man with a grim past, having spent three years in a salt mine. Luigi, Mario’s cheerful roommate, receives devastating news about his health, revealing he is terminally ill from cement dust exposure. Although Mario bonds with Jo over their shared experiences in Paris, Jo's aggressive demeanor creates friction with other patrons at the local cantina.
A significant disaster strikes when a fire breaks out at an SOC oil field, demanding the urgent transportation of nitroglycerin to extinguish the flames. This perilous task requires transporting the volatile substance in jerry cans over 500 km (300 miles) from the SOC headquarters, a journey made treacherous by deteriorating roads. Recognizing the risks, SOC deems the task too hazardous for their unionized workers and instead turns to the locals—for a siren's call of $2,000 each, a sum that could potentially liberate them from their tedious existence.
The selection narrows to four; Mario, Bimba, Luigi, and a German named Smerloff. Upon discovering that Smerloff fails to show up on the designated day, Jo steps in, having a prior connection with foreman Bill O'Brien from his bootlegging days. However, the other drivers suspect Jo might have used intimidation tactics to replace Smerloff.
As they embark, Mario and Jo handle one truck, while Bimba and Luigi take the other, with their vehicles separated by half an hour to minimize risks. Each driver faces a relentless series of daunting challenges: navigating through a stretch known as "the washboard," maneuvering around a precarious construction barricade, and clearing a boulder that obstructs their path. Tension arises as Jo finds himself succumbing to fears, prompting confrontations about his growing cowardice.
Tragedy strikes when Luigi and Bimba's truck explodes unexpectedly, claiming their lives. Mario and Jo rush to the scene only to discover a deep crater rapidly filling with oil from another ruptured pipeline. In their frantic efforts to prevent their vehicle from becoming stuck in the gooey mess, Mario accidentally runs over Jo, who sustains fatal injuries. Upon reaching the oil field, they are celebrated as heroes, yet the victory is hollow as Jo succumbs, and Mario, exhausted, collapses.
After recovering, Mario heads home in the very truck that was instrumental in their perilous journey, collecting double the wages of his fallen friends while refusing the chauffeur offered by the SOC. In a state of exhilaration, he recklessly navigates the mountain road, heading towards the celebratory atmosphere that awaits him at the cantina. As Linda dances inside, she collapses in distress. In a final act of defiance against fate, Mario takes a corner too swiftly and plunges off the guardrail, meeting a tragic end.
