It is late August of ad 79. The Roman world is reeling from the eruption of Mount Vesuvius. Volcanic ash covers the land, sunsets are blood-red, and the sea gives up corpses of the dead.

At a makeshift camp south of Stabia, hundreds of refugees from the cities around Vesuvius try to come to terms with the disaster. When children begin to go missing from the camp, Flavia Gemina and her friends Jonathan, Nubia, and Lupus investigate a powerful and charismatic man known as the Patron. A dangerous trail leads them to the caves and grottoes of Sorrento, where they encounter pirates, slave dealers—and death.