Montana Bill Bell (Kermit Maynard), a friend of the Three Mesquiteers, Stony Brooke (Robert Livingston), Rusty Joslin (Raymond Hatton) and Rico Renaldo (Duncan Renaldo), is killed in a rodeo accident and the trio reluctantly places his young daughter Peggy (Patsy Lee Parsons) in an orphanage while they earn enough money to qualify for her adoptation. Visiting the orphanage, they discover that Peggy's foot has been injured in an accident. The superintendent, Melloney (Byron Foulger), silences Peggy when she attempts to tell them about it. Despite their suspicions they agree to raise the $400 needed for special treatment required for Peggy's injury. To raise the money, Stony boxes four rounds in an exhibition with "Killer" McCully (Jack Roper) and succeeds in knocking him out. Taking the money to the orphanage, the Mesquiteers are impressed when they find that Melloney has stretched his budget and purchased the equipment already. They are unaware that Melloney and associate J. D. Crone (William Ruhl) did so to keep attention from being focused on the home, as they are exploiting the kids and taking funds alloted to running the orphanage. When Stony sees a small boy being bullied by an attendant, he hears enough to convince him that a child-labor sweatshop is being run in the basement. In the garb of a "Masked Rider", and with the aid of Rusty, Rico and nurse Ruth Miller (Loretta Weaver), the job of rescuing the children and convicting the crooks begins.

Stony Brooke, Rusty Joslin and Rico, The Three Mesquiteers, become parents when their friend is killed on the rodeo circuit and they promise to take care of his little daughter Peggy. When legal technicalities halt the adoption proceedings, however, the trio are forced to place Peggy in an orphanage while they earn enough money to qualify for adoption. While visiting Peggy at the orphanage, they discover that her foot has been injured, and Melloney, the superintendent of the orphanage, claims that the institution cannot afford to buy a special apparatus to treat the foot. In reality, Melloney and Crone, the director of public welfare, have been ciphoning the orphanage funds into their own pockets. To pay for Peggy's treatment, Stony fights a professional boxer known as "the Killer" and wins the exhibition money. When they visit Peggy to tell her the good news, Melloney silences the child and their suspicions are further aroused when they see a small boy being bullied by an attendant. After the trio leaves, Crone orders his thugs to ambush them, and when Rico recognizes one of their assailants as the hospital attendant, Stony dons a mask, and with the help of Ruth, a nurse, they raid the orphanage and take the children to their ranch. Melloney and his henchmen fail in their attempts to silence the mesquiteers before a hearing of the state commission in which Peggy testifies that her foot was injured when she was forced to carry heavy machinery. As Peggy testifies, Melloney and his gang break into the room and begin shooting, but they are subdued by the mesquiteers and several of the boys from the orphanage whom Rico has taught the art of wielding a bolo. With Melloney's conviction, Ruth is appointed the new superintendent of the orphanage and the children face a happy future.