In 1850, in Bordertown, on the Mexican-American border, Bart Breslow, the ex-convict owner of a Mexican cantina, looks forward to the arrival of noted performer Dolores Mendoza. In honor of her appearance, the American and Mexican authorities have agreed to open the border for Fiesta Day. Unfortunately, Dolores' arrival is upstaged when her stagecoach is held up by a masked bandit. The bandit, however, is Ted Cameron, a U.S. Marshal, who, along with his partner, Mortimer P. "Rockabye" Jones, is investigating a counterfeiting operation in the area. Saloon owner Ace Morgan, one of the leaders of the counterfeiting gang, overhears Ted pretending to threaten Rockabye, so he orders two of his men to kill the rogue outlaw. They are quickly captured by Ted, however, and taken back to town, where Ted demands that Ace offer him a partnership. Neither man knows that Dolores is a Mexican undercover agent who is working on an independent investigation of the counterfeiters with Captain Jose Garcia. Later, Ted crosses the border, where he meets with Bart, but the cantina owner tells him that he has given up his notorious crooked past. Soon, Ted is identified as the masked bandit by Marie Manning, another occupant of the robbed stagecoach, but he makes his escape from the Mexican authorities with the help of Rockabye, Dolores and Estrella, Dolores's elderly assistant. Later, Dolores sees Bart use a secret passageway which leads to the underground counterfeiting operation. She then overhears Bart and Marie yelling at Ace for his inability to handle Ted and their plans to transport another large shipment of counterfeit bills. Dolores is captured by the outlaws, however, before she can report their whereabouts, but her sudden disappearance leads Ted and Rockabye to investigate. Ted soon finds the underground hideout, but he is captured by Marie. After learning that Bart and Marie are married, Ted is rescued by Rockabye. Marie, in turn, is rescued by Bart, but their escape is short-lived, as the American and Mexican authorities combine their forces to chase down the counterfeiters. Captured on the Mexican side of the border, the outlaws are turned over to Captain Garcia, and Ted returns to Washington to arrange their extradition to the United States, but not without a quick stop first in El Paso to marry Dolores.
