Tyrone Power stars as 19th century nobleman Don Diego de la Vega, whose father, the mayor of Los Angeles, is removed from office by the sinister Captain Pasquale (Basil Rathbone), and a new corrupt "alcalde." By day, the crooked politicians rob the citizenry at every turn, and at night, Don Diego, wearing a mask, defies their laws and avenges the innocents. As he leaves his trademark - the letter "Z" - wherever he goes, he also leaves an impression on the mayor's niece, (Linda Darnell), who can't help but love the hero in disguise.