Unable to legally capture and sell a herd of protected wild horses, corrupt rancher Rance Macgowan uses his trained killer horse, Volcano, to substitute for the real leader of the herd and cause havoc and death among the ranches. With the government about to drop the restrictions on rounding up the herd, the Three Mesquiteers find themselves in the middle of the controversy after their friend, Sheriff Miller is killed by Volcano.

John Drury saves Duke, a wild horse accused of murder, and trains him. When he discovers that the real murderer, a bad guy known as The Hawk, is the town's leading citizen, Drury arrested on a fraudulent charge.

A man escapes from prison, then joins up with a gang of stage robbers while at the same time working as a deputy in a distant town, hoping to ultimately find the outlaw who killed his father during a robbery years ago.

When Shag Smith kills Jim's brother Bob, Jim and Thunder quit the rangers so they can cross the border and join Smith's gang. Jim's plan is to get the gang to cross back over the border where the rangers will be waiting.