Cosima Nolinas (Codi) trained as a physician, but decided during her residency to give up medicine. As the novel opens, she is returning to her hometown, Grace, Arizona, to teach high school biology and care for her physician father, Doc Homero, who is suffering from Alzheimer's disease. Her younger sister, Hallie, has just left for Nicaragua to help with agricultural development. Codi's journey back to where she grew up reinforces a sense of aimlessness which she attributes to the death of her mother when she was three years old, to the miscarriage of an unwanted pregnancy when she was fifteen, and to her father's remoteness. She intends her stay to be temporary.

But gradually she is drawn into the community. She restarts a relationship with Loyd [sic] Peregrina, the Native-American father--though she never told him--of the child she lost in high school. She joins the town's struggle against a mining company that has polluted the town's water supply and now plans to dam the river. As her father's condition deteriorates, she learns more about the history of his connection with the town and, by examining the results of a life-long study he has done on a genetic anomaly affecting children born to second-generation inhabitants of Grace, she learns that her own hereditary background is far more deeply rooted in the town than she had known.

Codi's narrative is interspersed with her father's confused but illuminating memories of her childhood, and with the letters she receives from Hallie, who has always been the motivated and determined sister. When Hallie is kidnapped and then murdered by the contras, Codi's first response is to run away once more, but in laying her sister to rest and telling Loyd about their lost child, she realizes that she has found her home and--in her fierce and practical education of the new generation of Grace adolescents--her purpose.