The novel follows the Cecil family's arduous journey by riverboat and wagon from Boone's Lick, Missouri, to Fort Phil Kearny in Wyoming. Fifteen-year-old Shay narrates, describing the journey that begins when his Ma, Mary Margaret, decides to hunt down her elusive husband, Dick, to tell him she's leaving him. Without knowing precisely where he is, they set out across the plains in search of him, encountering grizzly bears, stormy weather, and hostile Indians as they go. With them are Shay's siblings, G.T., Neva, and baby Marcy; Shay's uncle, Seth; his Granpa Crackenthorpe; and Mary Margaret's beautiful half-sister, Rose. During their journey they pick up a barefooted priest named Father Villy, and a Snake Indian named Charlie Seven Days, and persuade them to join in their travels.

At the heart of the novel, and the adventure, is Mary Margaret, whom we first meet shooting a sheriff's horse out from underneath him in order to feed her family. Forceful, interesting, and determined, she is written with McMurtry's trademark deftness and sympathy for women, and is in every way a match for the worst the west can muster.