When her father dies suddenly, 15-year-old Lexie McDonald is left an orphan in the Wyoming territory. Her brother, Callum, is on the run, accused of a crime he didn't commit, and plans are in place to send Lexie to live with an aunt in New York City. Rather than be separated from her beloved horse Cougar, Lexie disguises herself as a boy and sets out along the dangerous Pony Express trail to find Callum and clear his name.