Steven Bancroft, a young officer in the U. S. Calvary, is given the assignment of ensuring that dirty-work by agents of Spain, Mexico or Russian aren't going to keep statehood-for-California from becoming a reality. Bancroft uses his guns to settle any debates regarding international laws.

A train known as the Iron Mule is loaded with passengers, and starts off on its trip. Along the way, the train faces numerous obstacles and delays. The engineer is prepared for most of them, but the real challenges come when the train is ambushed by Indians.

Buster clowns around in a blacksmith's shop until he and the smithy get in a fight which sends the smithy to jail. Buster helps several customers with horses, then destroys a Rolls Royce while fixing the car parked next to it.