On his first night of watch, a security guard at the Cavalry Palace, which is currently featuring an exposition about human sacrifices, is atrociously murdered. The resemblance to the scenes of the old Aztec sacrifices is eerie. The murder weapon appears to be one of the old obsidian knives on display. The police attribute the wild murder to a criminal being pursued for other recent homicides, also involving barbaric mutilations. However, the investigator Martina de Santo changes course choosing a line of investigation that brings her to a very distinct place.