Marco Mondragãn, a genial lawman on the New Mexican frontier in 1782, fears that his young wife, Paloma, will be stricken by the approaching smallpox epidemic. When their Comanche friend, Toshua, rescues Anthony Gill, a white physician, from the desert, they strike the titular bargain: Anthony will inoculate Paloma and their neighbors, and Marco and Toshua will escort Anthony to a Comanche hideaway that he suspects harbors his kidnapped daughter.