Thirteen-year-old Joselyn hates and despises the Carson City Home for Unfortunate Girls, the orphanage into which she has been placed by her brothers after the death of their father. The boys joined the Gold Rush and went off to California, and Jo, loving the outdoors and especially horses, is left learning sums and needlework. Bor-ing! When the threat of an Indian attack has the orphans billeted in "safe" houses in town, Jo meets and makes friends with Sarah, a daughter of a Paiute chief. Encouraged by her, Jo determines to escape being sent back to the Home, steals some clothes and, disguised as a boy, gets a job as a rider for the Pony Express. Unfortunately, one of her fellow riders discovers that she is a girl and threatens exposure unless she helps him steal a consignment of money he's heard is coming through. With the help of some of Sarah's family, Jo manages to prevent the robbery, catch the thief and collect a reward big enough for coach fare to California as well as a grub stake to go panning for gold. She has triumphed over racism (her own distrust of all Indians), sexism (her society's), and crime, all in one fell swoop!