For fifteen-year-old Ellis Cady, life has gone quiet on her western Tennessee homestead. Her father and older brother left to sell horses to the army two years earlier and never returned. She watched her mother's health decline, finally succumbing to a broken heart. Her twin brother left in search of their father, and while he was gone neighbors moved out of their Quaker community, searching for peace ahead of the final sweep of war.

Ellis is left with nothing but the company of the remaining horses and the letters and journals she continues to write, trying to make sense of a desolate world. A small band of soldiers rides through to claim the last of the herd, and hope for the return of life as she knew it, evaporates like the mist on the river.

When the head-strong mare, Billie, returns, having escaped from the soldiers, Ellis takes it as a sign to leave. Disguised as a boy, for safety and comfort, she rides off to find her twin. Though war refuses to fade, Ellis stumbles upon an unlikely group of rescuers who teach her family is more than blood, and love has no limits.