Marc Charvein, Wells Fargo express messenger aboard a Southern Pacific train, and his old friend Carlos Sandoval need to find and stop the gang responsible for a series of train robberies. Speculating there is a connection between the gang and hand-minted gold coins being distributed to poor Mexicans by a caped nightrider, the two men set out to investigate. Meanwhile, their mutual friend Lucy Barkley is fired from her job as a Harvey Girl in Los Angeles when she violently resists the physical advances of a rich customer named Adolphus Grindell. Realizing shes been in love with Charvein for a long time, she travels to Yuma to find him. But Charvein and Sandoval have left for Mexico. When she overhears a telegrapher tell of a plot to kill Charvein and Sandoval, she rides to warn them. When she finally catches up, the three travel into Mexico where they intercept the masked nightrider. But then they, themselves, are quickly captured and brought into the lair of the gang. The outlaw leader is vowing personal revenge against his former partner, Adolphus Grindell, Lucys tormentor at the restaurant, whose gold shipments the gang has been robbing. Although the trio can now identify the thieves, will they ever escape to tell the law?