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.

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.

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'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.

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.

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.

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.

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.

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.

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.

The kidnappers send word that Anna Maria must meet them if she wants proof her father is still alive, but Diego warns her it is a trick. Romero argues that she must go, so she agrees to go with him, Garcia and Reyes.

Anita is still hoping to find her father, but Sergeant Garcia has decided she is nothing but a liar and wants to send her back to Spain. Only Diego believes her story, and he tells her she needs to supply some proof of her father's existence.

Don Alejandro remains firm in his wish to send Anita back to Spain and tells the girl to pack her bags. Gonzales arrives to take her to the ship and Anita gives Alejandro a goodbye kiss.

Basilio has begun to enjoy his stay in Los Angeles despite Zorro's interference. He is surprised at how well his war bonds are selling, for the landowners have decided to help support the Spanish government as much as possible.

Even with his stolen gold gone, Basilio plans to remain in Los Angeles as a wealthy man. He decides to do so in a way that will let him exert a hold over Moneta Esperon, the attractive daughter of a wealthy landowner.

The recovering governor is beginning to become irritated by his confinement to the de la Vega home. At times it seems as if the only thing that can pacify him is a music box. Almost as soon as it begins to play, the governor falls asleep. Knowing that the Rebatos are still trying to kill the governor, Diego uses the box to lull him to sleep then Bernardo moves him to safety.

Celesta Villagrana and her servant, Montez, are on their way to Los Angeles when a hooded robber stops them and steals her jewelry. The thief knocks the travelers to the ground then departs, leaving Celesta lying there unconscious.

Zorro tries to save Constancia de la Torre from an ill-considered elopement with Miguel Serrano, who only wants her family jewels.