Feeling the pressure from a flood of settlers, the old Spanish families of the newly-created State of California fiercely guard their ancestral properties. The Three Mesquiteers find themselves caught in the middle of the culture clash, following an incident between the newcomers and the Ortega family. Unbeknownst to all parties, Don Carlos Montoya, a supposed friend of the Ortegas, is stirring up racial tensions in an effort to terrorize the family into selling their valuable land to his mining company. When Tuscon's diplomacy threatens to spoil his scheme, Montoya frames Roberto Ortega for murder. As the beleaguered family makes plans to sell their property and relocate, the Mesquiteers begin to suspect that they've all been pawns in a deadly game of greed.
Prairie Pioneers was produced towards the end of the popular, long-running Three Mesquiteers series, which was marked by the fairly frequent turnover of the principal players. Robert Livingstone had already left the series, then returned, and was soon to leave a second time, to be replaced by Tom Tyler.