Six-Gun Gorilla is O’Neil, a gorilla. As a baby he is trapped in Africa and brought to the United States. In Colorado he is sold to Bart Masters, a prospector. Masters is a kind man and treats O’Neil well, and O’Neil grows up to love Masters. Masters teaches O’Neil how to dig, fetch firewood, haul up buckets of water, cook, clean, and load and fire a revolver. Unfortunately, Masters is murdered by bad man Tutt Strawhan, leader of the Strawhan Gang, for what Masters knows about "the great motherlode."

When O’Neil finds Masters’ body, he swears revenge. O’Neil straps on a bandolier and two six-shooters and begins tracking the murderers across a hundred miles of Colorado mountains and badlands. He picks them off one by one, meanwhile discovering a talent for holding up stagecoaches and using them to chase fleeing gunmen. O’Neil dies as he kills Strawhan.