In spring 1782, young Revolutionary War veteran Barnabas Locke and his Abenaki friend, the visionary Squando, reunite to cross the Mississippi and travel deep into the Great Plains. At the riverbank, they find their hired keelboat commandeered by the imperious young Spanish Isabella and her brother Ramon, who are fleeing the river pirate Joe Fontaine with a cache of jewels.

In a violent encounter, the two friends are linked with the fate of the Spaniards and their intractable pursuer, and later with an ancient Arapaho elder who invites them into the busy life of his tribe for a long summer of adventure—an attack by a Kiowa war party, a shaman’s exorcism, a warriors' initiation, a buffalo hunt. Each of them discovers profound rewards in tribal life, but Ramon oversteps by hunting for gold; and as Barnabas guides the Spaniards overland to safety in Santa Fe, they are apprehended by a skeptical Spanish captain, who does not believe their story until they are attacked by Comancheros, led by the pirate Joe Fontaine, and must fight for their lives through a night of fire and blood.

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