The novel's heroine, Mary, is a teacher who moves from Indiana to Colorado and marries a local cowboy named Jim. A successful rancher, Jim finds himself the subject of malicious and false rumors that he is a cattle rustler and is murdered by vigilantes while in the sheriff's custody, leaving his widow the "administratrix" of his estate. To find Jim's murderers and avenge his death, Mary disguises herself as a cowboy and infiltrates the gang responsible for lynching her husband.