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.

In the West, during the time of the Civil War, Pony Express rider Roy Rogers delivers the mail on the frontier. Both the Union and Confederate forces seek the allegiance of California, and complications arise when Brett Langhorne, a newspaper reporter from the South, and his sister Ann are duped by Lassiter, a former senator of the Confederacy, into helping him. Believing that Lassiter is working for the Confederacy, Brett becomes involved in his scheme to send forged dispatches that would dictate the movement of Union troops in California. In reality, Lassiter cares nothing for the South, as his only desire is to create a Californian empire for himself. To accomplish this, Lassiter teams with outlaw Luke Johnson to steal a shipment of gold vouchers meant for the Confederacy. When Brett learns of Lassiter's treachery, he is shot by Lassiter, but before dying, exposes Lassiter's plan to steal the gold shipment. Roy and his faithful horse Trigger then race to save the dispatches, and with the help of the Union army, capture the scoundrels.