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.

In order to help the escaped fugitive Torres elude Captain Monastario's surveillance of his home, Don Diego decides that he must propose marriage to Torres' daughter.

When Captain Monastario arrests the fugitive Torres' wife and daughter, hoping to catch Zorro trying to rescue them, he instead catches Don Diego's chief vacquero Benito, who is in love with the daughter Elena.

Learning that a group of disgruntled landowners plan to seize the cuartel and free the political prisoners, Captain Monastario sets a trap to capture them, including Zorro's father Don Alejandro.

After Zorro hides his wounded father in the secret cave, as Don Diego he must divert Captain Monastario and his soldiers who are searching for the old man.

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.

No sooner does the honest new commandante arrive than he is assassinated by a mysterious stranger who frames an elderly beggar for the crime and appears to be part of a larger conspiracy.

Sergeant Garcia receives a mysterious note from Zorro. The happy soldier reads that Zorro will give himself up. When he returns for his sword, it is gone, so he sadly returns to the cuartel without having captured his foe.

The King's tax collector and his nephew are stopped by conspirators, while on their way to Los Angeles. The conspirators threaten the men unless they cooperate, then they seize Morales' official documents.

A quiet day in the pueblo is abruptly changed when a stranger is killed by an arrow, apparently the victim of an Indian attack. Diego helps Sergeant Garcia inspect the body for clues and finds another mysterious eagle feather, this time on the fake Indian arrow. Other than that, there is no hint as to who the stranger was or why he was killed.

A peaceful protest by a group of peons upset with taxes turns violent when one of the Eagle's men throws a rock through the magistrado's window. Galindo uses this as an excuse to order severe measures against the peasants, a move that upsets Don Alejandro. The other landowners want to set up vigilante posses to punish the peons, but Alejandro argues on the side of reason.

Ramon Santil has just inherited his father's ranch and holdings, and some bad news. The magistrado tells him there's a high tax that must be paid at once or he will lose the land, and Ramon doesn't have the money to pay it. In desperation, he sells all of his cattle to Carlos Urista, a recent arrival in town whom Diego suspects as another of the Eagle's men.

The fake Ortega and the magistrado announce they have captured Zorro, and a cart holding a masked man is dragged into the town square. A public unmasking will be held at noon, and Diego fears that an innocent man will be charged with Zorro's "crimes". He also realizes it may all be a trap, so when he dons his costume and rides into the square to free the prisoner, he watches carefully for soldiers.

The plans of the fake commandante and the magistrado go awry when Ortega spots Diego talking to Rosarita Cortez, a childhood friend who has recently returned from the capital. Ortega fears she will recognize him from her journey, for it was during that trip that he murdered the real Ortega and assumed his identity. Bernardo overhears the two villains planning her death, and notifies Diego.

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.

The evil work of the Eagle takes a new turn when the commandante's wife, Raquel, visits Quintana and Fuentes and shows them an eagle feather, proving that she belongs to the conspiracy. She informs them the Eagle is furious over the loss of the gunpowder and that they must recover it if they hope to stay alive.

Sergeant Garcia and the other soldiers are amazed one morning when every musket seems to misfire. When they check their gunpowder, they realize it has been replaced with charcoal. A search of the cuartel reveals that all of the gunpowder has been stolen, leaving the post defenseless.

Raquel's ambitious plans for her husband appear to end when she receives a letter from him. She had been promised by the Eagle that he would be made governor of California, but her husband wants her to join him in San Diego, for he plans to sail back to Spain. Realizing the Eagle will not let her go, she tries to sneak out of the pueblo, only to be caught by Quintana.

Another agent sent by the Eagle arrives in the pueblo, this time smuggling jewelry to finance the conspiracy. Carlos Murietta, an Argentine who is contemptuous of the locals, particularly Sergeant Garcia, visits a tannery run by Jose Mordante. The two men discuss the disposition of the gems, including a famous piece known as "The Cross of the Andes", which were stolen from churches in Argentina.

Expecting another shipment of jewels, Murietta goes to inspect the tannery where they will be stored. There, he spots Pasqual, a peasant who lives there, and whips the old man until he leaves. The shipment arrives accompanied by Dolores Bastinado, recently arrived in the pueblo, who attracts the attention of Sergeant Garcia.

Determined to find the missing jewels, Diego and Bernardo visit a local padre in the hopes he can assist them in their quest, but the trip is to no avail. Later, when Carlos Murietta complains to Sergeant Garcia about the missing trunk he had stored at the tannery, Diego is convinced more than ever that the jewels were hidden by Mordante before his death.

Another shipment of stolen jewels arrives in the pueblo, and Dolores Bastinado unwittingly allows her brother, Pogo, to deliver the cargo to the Murietta brothers. The trunk slips and breaks open, and Pogo spots several of the gems. He realizes the men will try to kill him to protect their secret, but he is seized before he can flee. The Muriettas bind and gag him, then hide him when they hear Sergeant Garcia arriving.

Don Alejandro convinces all of the landowners to sign a document pledging their loyalty to the government, for he has become worried by the growing signs of possible civil unrest. He leaves a copy in the hacienda and takes the original to the capitol, intending to present it to the governor. Alejandro has barely left when Diego receives some unwelcome news. A new administrator has arrived in the pueblo and announced his decision to use the de la Vega hacienda as his headquarters.

An emissary from a foreign power has arrived in the pueblo to discuss plans to help the Eagle in his rebellion. George Brighton, the agent, warns Varga that there must not be any open resistance. His government wants an obedient colony, not one torn apart by internal battles. Varga promises to deliver control as planned, but he continues to worry about Don Alejandro and the other landowners who have pledged their allegiance to the King.

Diego overhears Varga telling his aide that the de la Vega hacienda is so comfortable he intends to make it his permanent headquarters. Knowing his father will soon return and would fight to the death to protect his home, Diego decides to try a risky plan to drive the Eagle out.

Another stranger arrives in the pueblo and travels to the de la Vega hacienda, intending to meet with the Eagle. Diego recognizes the visitor as Count Kolinko, an ambassador from an Eastern European country that has long been interested in controlling California. Since the Eagle has left for the pueblo, Diego manages to convince Kolinko that he is also part of the conspiracy. He almost succeeds in gaining necessary details of the plot.

Bernardo, who is keeping watch over Varga, is disturbed to see Garcia leading a patrol of lancers out of the cuartel. Even more disturbing is the sight of several strangers who drift into the Eagle's headquarters. When he tries to look inside, the gates are locked, so he rushes to warn Diego of the threat.