The monastery of San Giovanni on Rome's Aventine Hill has no real treasures, except for one huge and disturbing painting, dubiously attributed to Caravaggio, of the breaking of Saint Catherine on the wheel. It's not a subject likely to appeal to many buyers of stolen art. But a Caravaggio is a Caravaggio - or is it? Following a recent burglary at the monastery's chapel, there's little left to steal, so Flavia di Stefano of Rome's Art Theft Squad is particularly puzzled when she receives a tip that thieves plan to raid the building. What is there, except perhaps the Caravaggio, that professionals could covet? Even stranger is the sudden arrival in Italy of Mary Verney, an Englishwoman and thief whom Flavia and her art-expert fiance, Jonathan Argyll, have encountered before. She may be there as a tourist, but it's unlikely. Is Mary after personal riches, or is her trip, and her possible involvement in a theft, inspired by more terrifying circumstances? Something strange and threatening is occurring both inside and outside the monastery, and Jonathan and Flavia feel powerless when they fail either to stop a theft or a murder. As the two search for answers through the maze of monastic and police bureaucracy, they gradually reveal a surprise more shocking than even they had imagined.
