In 1870, a stagecoach is robbed of gold from the Lucky Seven mine, outside an Arizona town, and the guards and driver are killed. Sheriff Richard Baldwin, who is in cahoots with the robbers, and his cohort, Rufe Calloway, trail a passenger who escapes from the stagecoach. The passenger, Donald Clark, comes across Mary Jane Patterson, a rambunctious tomboy whose father was robbed and killed nearly a year earlier by an unknown assailant. While Donald hides, Mary Jane misleads the sheriff, then evades Rufe when he tries to take her to school. Mary Jane sneaks up on her guardian, Manuel Hernandez, who is driving a team of horses, and causes the wagon to spill over. As they retrieve the strewn boxes, Mary Jane finds a dugout cave and a box from the Lucky Seven mine hidden there. Manuel returns the box and, with Mary Jane's assistance, tells the sheriff and townspeople a tall story of how he subdued the masked bandits. After Manuel forces Mary Jane to go to school, she fights a boy who makes fun of Manuel. That night, when Manuel learns of the fight, Mary Jane, to avoid hurting his feelings, says that she was taunted because she dresses like a boy. Manuel's wife suggests that Mary Jane wear her wedding dress, but when Mary Jane looks for it, she finds a wanted poster for Manuel, who is called El Gato in the poster. She asks Manuel about El Gato, and he tells her that ten years earlier, the bandit El Gato and his band robbed from the rich and gave to the poor. When Donald comes to see the sheriff, he recognizes Baldwin as the man he has been trailing, who, five years earlier, killed his father in Virginia and subsequently killed three men in Dodge City. Baldwin shoots Donald, but Donald escapes and rides to Manuel's home, where Mary Jane's schoolteacher, Caroline Reid, is visiting. When Baldwin and his men arrive, Donald, not wanting to involve Caroline, asks Manuel to turn him in. For his protection, Caroline, Manuel and a judge ride with the sheriff to accompany Donald to jail, where he is held for the stagecoach murders. Mary Jane throws rocks with messages from El Gato through the windows of Manuel's former cohorts, alerting them to prepare to ride again, and one through Baldwin's window ordering him to release Donald. After Baldwin makes an insulting remark about El Gato and Manuel learns of Mary Jane's manuever, he struggles to get into his El Gato costume, having gained a considerable amount of weight in the interval, and meets with his men. That night, El Gato and his men race through town shooting up the streets and rescue Donald. The sheriff and his men are unable to follow because Mary Jane has cut their stirrups and tied their saddles to posts. Baldwin, however, arrests Manuel, but during the trial, Mary Jane throws bullets into the stove in an adjoining room. After the subsequent explosions clear the courtroom, Mary Jane finds the Lucky Seven mine box hidden under the floor of Baldwin's office. Although Baldwin and his gang try to escape, they are stopped by Manuel, as El Gato, Donald and Mary Jane. Manuel is made the new sheriff and Mary Jane his honorary deputy.