In 1244, at the infamous fortress of Montségur in southwestern France, Christian Inquisitors massacred the last vestige of a heresy known as Mithraism, once the dominant religion of the Roman Empire. But was the heresy in fact destroyed? In today’s New York City, reporter Tess Drake discovers a mysterious altar in the apartment of a friend whose charred body was discovered in a nearby park. The altar (a version of it exists in the British Museum) depicts a man astride a bull, plunging a knife into its neck while a dog, a scorpion, and a snake drink the blood. The meaning of that altar propels Tess on a furious hunt for the truth about what happened to her friend, a labyrinth of intrigue and danger that leads to the highest levels of power as well as to the depths of a secret cave in Spain.