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After Henrietta Denton, heiress to the Norris Trucking Company fortune, is killed in an automobile accident on Highway 13, her widower Frank, a Norris employee, hires private investigator George Montgomery to investigate his wife's crash as well as a series of recent accidents involving Norris trucks. Later, while driving down the highway, Hank Wilson, a Norris trucker who witnessed Henrietta's accident, decides to visit the Clover Café truck stop run by his girl friend Doris' aunt Myrt and uncle "Pops" Lacy. Doris, who works as a waitress at the café, presses Hank to ask the company for a raise. Before he leaves, Pops, who is a mechanic on a retainer from the company to inspect its trucks, examines Hank's vehicle and assures him that it is in good working order. Hank then returns to company headquarters and asks personnel director Mary Hadley for a raise. Denton questions Hank about Henrietta's accident, and after Hank recounts what he saw, Denton thanks him and instructs Mary to give him a raise. Later, after a sixth Norris truck crashes, George goes undercover as a driver, and Mary asks Hank to take George along on a couple of runs so that he can familiarize himself with the route. After driving for a spell, the two men decide to stop at the Clover Café. While there, Pop checks Hank's engine, then he and George resume their journey. Later, George tells Denton that he suspects that Pops, a former gangster, may be responsible for the sabotage the company is experiencing. Soon, George is assigned to his own truck, and after a stop at the Clover Café, the truck's brakes fail and he is almost killed. Meanwhile, Hank and Doris become engaged. One night, in the company's yard, Hank parks his truck at a loading dock and leaves while George checks a load. Suddenly, someone releases the hand brake on Hank's truck and it rolls backward, crushing George to death against a dock. Later, Hank, who was nearby when the killing occurred, becomes the prime suspect, loses his job and is arrested on suspicion of murder. In jail, however, he receives a visit from Kelleher, an attorney representing Norris' insurance company, who tells him that George had told them that he, Hank, was in the clear. Kelleher then offers to arrange to get Hank his job back if he will help him find out who is behind the deaths and sabotage. Hank agrees and reports to the office where, to his surprise, Mary flirts with him and they kiss. The next time Hank visits the café, word of his dating Mary has reached Doris and, although Hank declares his innocence, Doris breaks their engagement. Later, however, Hank explains his undercover assignment to her and tells her about her uncle's criminal past. After Doris tells Pops about Hank's accusations, he phones Mary and tells her he is quitting. Hank overhears the call and tries to take Pops to the police, but Pops knocks him out with a bottle, then ties him up and puts him in the front seat of his panel truck. Doris witnesses the attack and sneaks on board the back of the truck. Hank recovers consciousness and sees Doris behind the glass partition, although Pops is unaware of her. When the truck comes to a halt, a convertible with Denton and Mary pulls in beside it. They are secret lovers who have already disposed of Henrietta to gain access to her fortune and intend to collect insurance money resulting from the company's numerous disasters. They plan to kill not only Hank, but also Pops, who has been their saboteur and then blame them for the accidents. Denton slugs Hank again, rendering him unconscious, and Pops send the driverless truck on its way down a slope. However, Doris breaks the window between the front and the back of the truck and grabs the wheel. Meanwhile Denton and Mary run over Pops with their convertible. As the truck careens down the highway, Doris and the recovering Hank manage to wrestle it to a halt. When Denton and Mary swerve to avoid the truck, their car crashes over an embankment and bursts into flames. The police and Kelleher arrive, having been tipped off by Aunt Myrt, and Kelleher indicates that Hank and Doris can expect a substantial reward, which will help them plan their life together.