Border patrolman Tom Sharper stops a truck carrying soybean oil just outside the border between the United States and Mexico, but as he attempts to inspect one of the barrels, the drivers knock him out and hide his unconscious body. Meanwhile, cattle driver Roy Rogers, returning with his herd from Mexico, arrives at the border, and, seeing that his friend Tom is missing, continues on after leaving him a note. The truck drivers then proceed to their meeting point, where they unload their real cargo, American criminals who have been kept in Mexican prisons, and inform their boss, Bart Carroll, that they lost one of the barrels and knocked out a border patrolman. Distressed by the news, Bart loads the men back into the barrels, drives them to a cliff over a reservoir, and sinks the barrels in the water. Later, Bart sends a carrier pigeon to his boss, store owner Willis Newcomb, and tells him that his gang knocked out Tom. Newcomb orders Bart to lay low, as he is concerned about Tom's father Alf, an enemy ex-marshal who once ran Newcomb out of town. Soon after, Roy is questioned about Tom's disappearance by Judge "Cookie" Bullfincher, and Alf and Newcomb arrive just in time to hear Roy tell Cookie about the oil truck he saw. In town later, Alf, despite his failing health, fights Newcomb after hearing him imply that Tom ran out on his job. Newcomb then orders Bart to find Tom, at the same time that Roy organizes a posse to search for him. Bart finds him first, and uses Tom's gun to kill rancher Anderson, who is taking Tom back to Alf. He then tells Tom, who is suffering from amnesia, that Tom killed Anderson and now must stay with the gang for protection. Soon after, Roy finds Anderson's body and Tom's recently shot gun, a discovery that arouses suspicion that Tom killed the rancher and ran. When Alf hears this news, he collapses. Later, Newcomb learns that one more shock will kill Alf, and so sends a carrier pigeon to Bart, instructing him to rob a bank and make it appear to be Tom's fault. He then invites Roy and Cookie to town to ensure that they witness the robbery, and as Tom flees the scene, Roy captures him. No one believes Tom's story that he has amnesia until Roy revives his memory by knocking him out again. Tom tells him about the oil truckers, recalling that one barrel fell off before he stopped them, and when Roy and his men recover the barrel, they find a dead criminal inside and deduce the smuggling scheme. That night, Roy and Cookie investigate Newcomb's store and steal a carrier pigeon. The next day, they wait for Newcomb to send another pigeon to his henchmen, then shoot down the bird in order to read its message, which states that another shipment of oil is waiting at the same place. They send the message on the stolen pigeon and wait at the border for the oil truck. When it arrives, Roy and Tom hold up the truck, but Newcomb and Bart jump out, attack them and shove them into the oil drums. Roy manages to rock his barrel off the truck and hops onto his horse, Trigger, but is stopped when the gang sinks Tom's barrel in the reservoir. Meanwhile, Cookie's posse attacks Newcomb's truck, and when Roy catches up with them, he fights with Bart until the outlaw falls off a cliff. Alf then wins a fight with Newcomb, but staggers when he hears that Tom is dead. At that moment, they hear a man inside one of the barrels and open it to find Tom, who tells them that the gang rolled the wrong barrel into the water. He greets his father as the sheriff prepares to bring the gang to jail.