Montana Bill Bell, a friend of the Three Mesquiteers, Stony Brooke, Rusty Joslin and Rico Renaldo, is killed in a rodeo accident and the trio reluctantly places his young daughter Peggy in an orphanage while they earn enough money to qualify for her adoption. Visiting the orphanage, they discover that Peggy's foot has been injured in an accident. The superintendent, Melloney, 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 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 did so to keep attention from being focused on the home, as they are exploiting the kids and taking funds allotted 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, the job of rescuing the children and convicting the crooks begins.