A mystery set in ancient Rome. When Flavia, Jonathan, Lupus, and Nubia rescue an admiral from a boating accident, he not only rewards them, but he also offers them a riddle that holds the promise of great treasure to anyone who can solve it. He explains that he saw the riddle on the wall of a blacksmith shop and that the blacksmith could help solve it if they can find him. Later, the four friends spend the summer with Flavia's uncle, who lives near Mt. Vesuvius in Pompeii and find Vulcan, the blacksmith. He is an orphan and wants to solve his own mystery of why his parents abandoned him. Just as they are all beginning to figure out these conundrums, the great Vesuvius erupts and they must flee for their lives. The action begins slowly and doesn't really pick up until the volcano starts erupting, about halfway through the book. Also, the mystery element is somewhat confusing and hard to follow. However, the many interesting details about Roman life incorporated into the story provide insight into the period.