In the midst of the Civil War, Lassiter has a plan to get control of California. Working out of St. Joseph, he plans to send forged messages to the troops on the west coast via Pony Express. First he attempts to bribe Pony Express ride Roy Rogers. When Roy refuses he turns to the outlaw Johnson and his gang and this leads to trouble.

"Brave men on gallant horses...risking life and limb on the western plains...to insure the safety of their precious cargoes."