Diamonds are being smuggled across the border from Mexico in a specially made shoe of a palomino mare. One of the smugglers is killed when the mare runs off. The sheriff blames Trigger for the death. To keep his horse from being destroyed, Roy confesses and goes to jail. The smugglers buy Trigger and put him to work smuggling diamonds. The mare, who had earlier heard a trist with Trigger, foals Trigger, Jr. who Roy, finally out of jail, uses to help capture the smugglers.

The search for a lost gold mine hinges on a secret branded onto the neck of a great horse, White Fury.