A swindler forges documents to make himself the owner of an entire state.

In 1912, during a party at the governor's mansion to celebrate Arizona's admission to the Union, John Griff, an employee of the Department of the Interior, tells the story of the notorious James Addison Reavis, a clerk working at the land office in Santa Fe, New Mexico, who almost "stole" Arizona: In 1872, Reavis visits the home of Pepito Alvarez, a Mexican living in Phoenix. After telling Pepito that he works in the federal land office, Reavis inquires about Sofia, an infant whom Reavis left with Pepito some years previous. When Reavis meets Sofia, now a shy young girl, he tells her that he is going to bring her to live with him in Santa Fe. Reavis, who is plotting to turn himself into a phony baron, wants Sofia as his baroness and hires governess Loma Morales to school her in proper etiquette. In the meantime, Reavis spends long hours chiseling his forged land grant into a stone tablet which he will use to establish the claim of his fictitious "first baron of Arizona," Miguel de Peralta. Later, Reavis purchases some headstones to establish the births and deaths of Sofia's "parents" and then sails to Europe to complete the forgery. In Spain, he joins a monastery at which the original land grant book of King Ferdinand VI is kept. Taking the name Brother Anthony, Reavis waits three years for an opportunity to be left alone in the library so that he can alter the grants record book, giving his baron the territory of Arizona. During this time, Reavis learns from the monastery's head, Father Guardian, that a second copy of the book is kept at the Madrid castle of the Marquis de Santella. When Reavis is finally able to commit the forgery, he is caught by Father Guardian and forced to flee. Reavis steals a horse-driven cart, but it overturns along the winding monastery road. He is rescued by a band of gypsies, then travels to Madrid, and after befriending the Marquesa, convinces her to invite him to the castle. There, Reavis alters the entry in Santella's book, returns to Arizona, marries Sofia and claims Arizona. Suspecting forgery, the surveyor general, Miller, investigates the claim, which is validated by the discovery of the tablets. When Reavis begins evicting the residents of Arizona, Griff, an expert in the art of forgery, is consulted. After the local newspaper criticizes Reavis, a displaced landowner, Tom Lansing, firebombs his office in town. Reavis then rejects a government offer to buy the territory for $25,000,000, and he and Sofia are summoned to federal court to defend their claim. Griff states that he cannot prove that Reavis' claim is false, but later, Pepito tells Reavis that he plans to break his silence and tell Griff about Sofia's real parents, who were Indians. Reavis attempts to flee the country, but Griff arranges for an immediate local trial. After Pepito is shot by an angry mob, Reavis is nearly lynched, but saves himself by reminding the displaced landowners that they need his testimony in order to reclaim their land. Years later, when Reavis is released from prison, he is surprised to see his faithful wife, a now recovered Pepito and Loma waiting to take him home.