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.

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.

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.