In the spring of 1876, when two Colorado deputies return to the tree they tied their horses to before chasing a fugitive into a forest, the horses are gone, without a trace. As they walk back to town, leading their bound prisoner, they discuss the possible reasons for their predicament. Xavier Garza, the half-Zuni deputy, believes he knows the answer: Atahsaia--a Zuni word meaning cannibal demon. His partner, Thomas Wilkins, scoffs at such superstition, until the mounting evidence changes his perspective on monsters--and on trust.