Christmas Eve is no holy night for Boston Medical Examiner Maura Isles as she is summoned to the scene of a vicious killing. Teamed up with Detective Jane Rizzoli, Isles links the unfortunate victim to Joyce O'Donnell, the confrontational psychologist who so frustrated their earlier pursuit of the notorious murderer known as the Surgeon. Isles and Rizzoli follow O'Donnell's trail to a sinister cabal called the Mephisto Club.

On Beacon Hill, the bookish members of the club devote themselves to the analysis of evil. But with the grisly appearance of a corpse on their doorstep, they begin to fear the very subject of their study. Could this maniacal killer be among them? Or have they inadvertently summoned an evil presence from the darkness? To unravel the macabre mystery, Maura and Jane embark on a labyrinthine journey to uncover the ancient roots of evil, along the way facing a malevolent foe more dangerous than any they've ever faced.