When bright, brash Jessamine Murphy finds a recruitment poster for the Pony Express, her tomboy heart skips a beat: not only for adventure, but for the chance to track down her wayward father in California. Eager to reunite her fractured family, Jessamine cuts her hair, dons a pair of trousers, and steps into the world as Jesse.

With a bit of trickery, Jesse wins a special assignment-as does Ben Foley, a quiet but determined boy who guards secrets as closely as Jesse does. The two are to transport unusual cargo along an unusual route: the Nightland Express. They ride west together, one excitedly navigating the world as a boy, the other passing as white to escape the monsters from his past.

Ben and Jesse soon realize their assignment is special in more ways than one: their tireless horses cover ground faster than should be possible, and inhuman creatures watch their journey from the darkness. The Nightland Express is more than a mail route-it traces the border between the mortal world and a vibrant, magical land just beyond.

As both realms hover on the precipice of disaster, Jesse and Ben must learn to fully trust one another before a catastrophic rift separates the two worlds-and the two riders-forever.