It's the feast of Saturnalia when all the world's turned upside down with dancing and feasting and mis-rule. An abandoned ship has washed up in Ostia - carrying ghosts or monsters or who knows what. And Flavia has another challenge when her father brings home a horrible woman he wants to marry.

Nubia tracks down the monster that escaped from the ship - and discovers there's more than one monster and nothing is what it seems. Flavia's battle to expose her father's new fiancee as a murderer gets her into deep trouble and finally, comes close to costing her life and the love of her friends.