Henry sets his novel during one of the many despicable periods in US history- the massacre of the Cheyenne at Sand Creek. He includes details of the massacre that he gleaned from the military report that resulted from the investigation into the mass killing. Those details were difficult to read.
As for the fictional portion of the book, it describes a white man's efforts to save and educate Indian orphans. Because of these efforts, Nehemiah Bleek runs afoul of Colonel John Chivington who was the officer who led the troops who committed the Sand Creek Massacre. Henry paints a picture of Chivington as a mentally ill man. My reading of historical reports about Chivington indicated that may have been mentally ill and he sure hated the Indians. ( )