The author's journey begins with a simple question: why do humans, unlike other mammals, seem to struggle with running injuries? This leads him to the legend of the Tarahumara, a reclusive Mexican tribe renowned for their incredible running abilities. He reads about a mysterious figure called Caballo Blanco (the White Horse), a chalky white man who lives among them, shrouded in rumors of being a fugitive or a self-punishing boxer. Intrigued, the author travels to Mexico to find the Tarahumara and Caballo.

He teams up with Salvador Holguín, a local who knows the treacherous Copper Canyons where the Tarahumara reside, a region also plagued by dangerous drug cartels. Their journey is fraught with peril. They eventually reach Ángel Nava López, who runs a Tarahumara schoolhouse and offers insights into Caballo Blanco. Ángel describes him as a strange but kind outsider who has lived with the Tarahumara for years and understands their ways. The author suspects the White Horse story might be a way to deter outsiders.

Despite the challenges, the author presses on and finally encounters Caballo Blanco in the town of Creel. He learns that Caballo's real name is Micah True, and he's an eccentric individual with a background in boxing, originally from Colorado. Their initial meeting is marked by Caballo's intense hunger after a long, unplanned run. Caballo reveals his audacious plan: to organize a race that brings together Tarahumara runners and elite athletes from the outside world. He has even started contacting runners like Scott Jurek via a slow dial-up connection from a remote candy shop. The author finds himself increasingly drawn into Caballo's vision.