While working as a groundskeeper at a convent in Ohio, former slave Mary Fields forms a deep friendship with the superior, Mother Amadeus Dunne. When Mother Amadeus falls ill after traveling to Montana to open a girls' school, Mary rushes to her rescue and nurses her back to health. Mary soon discovers that the Western Frontier satisfies her free-spirited nature, and she decides to stay in Montana for good. She secures a contract to deliver mail by stagecoach for the U.S. Postal Service, becoming the first African American woman to carry mail. With a fearless attitude and multiple firearms, she protects the mail from would-be bandits and is soon known far and wide. Some years later, legendary gunfighter-turned-newspaperman Bat Masterson meets Sister Amadeus and gets the true story of "Stagecoach Mary."