The Latin 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 controversial celebrity psychiatrist Joyce O'Donnell - Jane's professional nemesis and member of a sinister cabal called the Mephisto Club. On tony Beacon Hill, the club's acolytes devote themselves to the analysis of evil: Can it be explained by science? Does it have a physical presence? Do demons walk the earth? Drawing on a wealth of dark historical data and mysterious religious symbolism, the Mephisto scholars aim to prove a startling theory: that Satan himself exists among us. With the grisly appearance of a corpse on their doorstep, 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 this maniacal killer be one of their own - or have they inadvertently summoned an evil entity from the darkness? Delving deep into the most baffling and unusual case of their careers, Maura and Jane embark on a terrifying journey to the very heart of evil, where they encounter a malevolent foe more dangerous than any they have ever faced . . . one whose work is only just beginning.