Young, poor, and orphaned in rural Missouri, Jessamine Murphy is fretting over her very pregnant sister, not at all sure how she's going to feed her family until the baby is born, let alone after. When she comes across a recruitment poster reading "Pony Express Special Assignment: St. Joseph, Missouri to California. Two riders wanted. Orphans preferred," her heart skips a beat: not only for the ample risk wage, but for the chance to track down her wayward father in California. She cuts her hair, steps into the role of Jesse, and heads down to the Pony station.
Once there, she meets Ben Foley, a quiet but determined boy, so secretive about his origin story there is little doubt it must be turbulent. They are set up as partners, an odd pair—one who is increasingly unsure whether she is (or ever was) a girl, the other fighting for his place as a mixed-race young man in antebellum America—yet their esteem for each other grows as they make the trip west across the United States.
At first the oddities can be explained away: a shortcut through a mysterious portal that carries them miles in an eyeblink; unusual creatures with uncanny glowing eyes. But these are just early portents of a bigger struggle, evidence of a darker and more magical realm that is on the brink of cataclysm even greater than the human realm's.
As Ben and Jesse struggle to find themselves and save their world, they discover their unlikely alliance may be the only thing that will save them . . . and the fate of the creatures and land of an entire world beneath our world.