After a long hiatus following the death of his wife, Annie Blue, sometime Pinkerton Agent, Deputy US Marshal and freelance detective Lucas Santana is once again called to serve.
The smooth-talking, well-read Wyoming private eye is ordered south to Riverton County Texas, to investigate the rumoured growing unrest there.
Washington is worried that the ambitions of one man could destroy the peace on that lonely stretch of borderline. The ambitious Frank Vagg controls the local law on both the Mexican side and the Texas side of the Rio Grande, straddled as it is by his headquarters, the township of San Pedro.
Santana's attractive contact, Henri Larsson, wary at first of the senior operative with the reputation for action, proves to be more of a match than he would like.
Santana attracts trouble like horse manure attracts dung beetles and it isn't too long before he is compelled to use his big Colt.
When the lead begins to fly he is joined by fellow Pinkerton agents Joshua Beaufort and Jacob Benbow and the body count grows in the grim, grey borderline county where no ravens fly.