The Latin phrase is scrawled in blood at the scene of a young woman's brutal murder: I HAVE SINNED. It's a chilling Christmas greeting for Boston medical examiner Maura Isles and Detective Jane Rizzoli, who swiftly link the victim to the sinister Mephisto Club, a cult of scholars who aim to prove that Satan himself exists among us. Then, with the grisly appearance of another corpse, it's clear that someone--or something--is indeed prowling the city. Soon the members of the club begin to fear the very subject of their study. Could the maniacal killer be one of their own--or have they inadvertently summoned an entity from the darkness? Delving deep into the most baffling case of their careers, Maura and Jane embark on a terrifying journey to the very heart of evil.