In Washington, Tom Kayler, an investigator with the U.S. Food and Drug Administration, is assigned to go to Los Angeles to discover who is illegally supplying the drug amphetamine to long-distance truck drivers, who use it to help them stay awake at the wheel. Overuse of the drug, known in pill form as “bennys,” induces hallucinations that have resulted in several fatal accidents. In Los Angeles, Tom poses as a student truck driver and moves into a boardinghouse run by Val Owens, the young widow of one of the dead drivers. When Tom meets hipster Mink Reynolds, a fellow roomer who works for Bodmer Freight Lines, the company that has hired Tom, Tom observes that Mink is obviously under the influence of some form of stimulant. The next day, Tom is assigned to go with old-timer Wally on a run to Portland, Oregon. While still in California, Wally makes a routine stop at Dunc Clayton’s service station, then continues the journey. As they drive, Wally warns Tom about “bennys” and tells him how they have just caused another trucker’s fatal collapse. After Tom says that he thought the pills were only available by prescription, Wally tells him that they are very easy to obtain and although he admits to using them, Wally says that he intends to find out who is behind their supply and put them out of business. Later, at the Six Points truck stop café, Tom and Wally encounter Mink, who is “higher” than before, and tells Tom that waitress Amy Phillips can supply him with bennys, although she later denies it. Between trips, Tom returns to the boardinghouse and meets Val’s old friend Steve Hummell, who is visiting her. Tom and Wally are on another run when Wally stops at the same café and gets out of the truck while Tom sleeps. Wally is then beaten to death by two thugs who have been following the truck. Later, when Tom and Val return from Wally’s funeral, Val asks Tom to keep her company and a romance develops betweeen them. Tom is next assigned to drive with Mink, and when they stop at the café, Tom tells Amy that he suspects that she may have been involved in Wally’s death as he was trying to expose the benny suppliers. Amy, who is also addicted to the drugs, promises to tell Tom the name of her supplier on his return trip. However, when Tom returns, the café owner tells him that Amy left suddenly. Although Val and Tom continue their involvement, Hummell proves to be a frequent, puzzling visitor. Tom is alarmed when, one day, Mink shows up at work after a night of partying, and insists upon driving while swallowing several bennys. After nearly having several accidents on the highway, they arrive at the café, where Tom is handed a letter from Amy, in which she names her supplier as a “Mr. Brown” who works with a prominent pharmaceutical company. When the café owner’s wife refuses to dance him, Mink goes berserk then later falls asleep in the cab when Tom and he drive off. After Mink wakes up, hallucinating, and tries to stab Tom, they struggle for control of the truck until Tom overpowers him. Tom then takes Mink to a local hospital where Mink recovers, but remembers nothing of the past few hours. After a repentant Mink tells Tom that Dunc is his supplier, Tom sets a plan in motion to entrap Dunc. One day, Val, curious about Tom’s background, enters his room and goes through his papers, discovering Amy’s letter and Tom’s real profession. Later, Tom is surprised when confronted by Val, Hummell, Dunc and a gun man. After Tom accuses Hummell of being “Mr. Brown” and Val of being his accomplice, the men then take Tom to a remote area, intending to kill and bury him there. When Tom convinces Dunc that Hummell will probably also kill him, as he knows too much, Dunc assists Tom in overpowering the other two and, after shooting Hummell, Dunc surrenders to Tom. When Tom returns to the boardinghouse, Val tries to persuade him that they can still be together, but Tom resists her appeals and tells her he intends to turn her in. Val attacks Tom then runs out of the room into the arms of two police officers.