Set in the mid-19th century American West, the story takes place in 1863 during the westward expansion. Martha's family, traveling with a wagon train, is so poor they possess only the bare essentials carried in their wagon. From a young age, Martha has been fascinated with riding, driving wagons, and adventure. She doesn't fit in with the other girls and is mocked by the boys for practicing physical labor. When her father is severely injured while breaking a horse, Martha must take on the responsibility of driving their family wagon and caring for the horses. After being bullied and getting into fights with boys, Martha cuts her hair short, puts on trousers, and decides to compete with the boys on equal footing. However, when the wagon train is robbed, the angry settlers quickly point fingers, convinced that this rebellious little girl has brought the thieves and calamity upon them. Martha deduces that the thieves couldn't have gone far. To clear her name, she sets out alone on horseback, embarking on an adventure disguised as a boy. Meanwhile, the pioneers' wagon train is about to traverse the most dangerous uninhabited territory—a place from which no one has ever returned alive—and they are in desperate need of a guide familiar with the terrain. Unbeknownst to Martha, the skills she hones through a series of perilous encounters will face even greater trials in the future.