The U.S. Government sends an undercover-agent to California in the days when American land-thieves were preying upon Sanish families holding rich land-grants from the Spanish Crown. Don Carlos del Valle, who has a beautiful sister, Rosa, has a grant that also has a gold mine, and land-grabber Owens plans to get it. The agent learns of Owens' plans and shows up at a masquerade ball, masked-and-cloaked, to warn Don Carlos. Owens and his gang show up with a forged land grant but the agent saves Don Carlos by killing one of the henchmen but is unable to prevent his assassination. In the aftermath, Rosas loses the hacienda and holdings and blames the Masked Stranger. Since no one has seen his face, he holds onto his masquerade costume and sets out to save the property for Rosa.