The story of the masked rider in Old California opens as Don Diego de la Vega is returning to California from Spain, where he has been studying for several years.

It is a calm morning in Spanish California, and Sergeant Dimetrio Lopez Garcia is putting up posters, with another soldier, which are asking for reward money for Don Ignacio Torres and the now famous Zorro.

Finding that Don Torres is in hiding at the Mission, Monastario forces the local Indians into hard labor until the Padre agrees to surrender the fugitive.

After failing in his attempt to sneak food to the fugitive in the Mission where Captain Monastario has laid siege, Zorro tries to scare the soldiers away by spreading a rumor about the mad monk who supposedly haunts the place.

With his father Don Alejandro now charged with treason along with Don Torres, Zorro must see that the judge sent by the governor arrives to assure a fair trial.

Captain Monastario orders Sergeant Garcia to pretend to have been dishonorably discharged, so that the people of the village will take him into their confidence and reveal Zorro's whereabouts.

Captain Monastario's latest plan is to discredit Zorro by having a prisoner who killed a man in a sword fight pose as the masked avenger and rob the local landowners at a dinner party.

Faking Martinez's death, Captain Monastario again tries to use him to impersonate and disgrace Zorro by robbing a church, leaving Zorro confused as to who could be the impostor this time.

Captain Monastario finally realizes that Don Diego must be Zorro, and arrests him, but a Viceroy from the King of Spain who is visiting the pueblo turns out to be an old friend of Diego's, and this is just enough to allow Zorro to turn the tables and have Monastario discredited.

Another stranger with an eagle feather spells trouble for Zorro when Perico, a recent arrival, sees a gypsy girl trying to pay for a purchase with several gold nuggets. Diego and Alejandro realize that news of a gold discovery will cause a rush of "gold fever" and their peaceful way of life will be threatened, so they try to suppress the news.

The magistrado's plans to aid the Eagle may suffer a setback when a new commandante arrives in the pueblo. Unlike the fake Ortega, Capitan Toledano is loyal to the King and beyond corruption, yet he must be eliminated if the Eagle's plot is to succeed. Toledano's jealousy over his pretty wife Raquel is the key to the magistrado's newest scheme.

Magistrado Galindo has smuggled a supply of weapons into the pueblo, intending to use them when the Eagle orders the overthrow of the government, but he has been unable to get a supply of gunpowder. The army has a large stockpile at the cuartel, and the magistrado decides to steal it for his use

Capitan Toledano plans to interrogate two prisoners who were seized when the gunpowder was recovered. The prisoners warn Figueroa, one of the magistrado's men, that they will talk unless someone helps them escape. Instead, they are found dead in their cell the next morning. A doctor examines the bodies and suspects murder, but the magistrado announces that it was suicide.

With the death of the magistrado, the Eagle takes a more active role in his conspiracy to rule southern California. He sends two of his men, Quintana and Fuentes, to Los Angeles with a load of gunpowder for the weapons previously smuggled into the pueblo. The two men earn the wrath of the Eagle for suggesting that Zorro may upset the plan, and they fearfully head off on their mission.