Christmas Eve in Boston is no holy night for medical examiner Dr Maura Isles.

In a rundown house a woman has been dismembered in an act of carnage that leaves veteran cops in shock. The last person called from the dead girl's phone is Dr Joyce O'Donnell, a celebrity psychiatrist who's made her name defending serial murderers.

But there are other clues that make the police wonder if this slaying was part of a Satanic ritual. Drawn on the wall, in blood, are ancient symbols, and a mirror-image word in Latin that, translated, says: "I have sinned."

Then a second woman is found butchered on Beacon Hill, just outside the grand residence of Anthony Sansone, a reclusive historian. He is the leader of the Mephisto Club, an old and secret society dedicated to the study of evil, and to confronting it in its purest form. On the door to Sansone's house have been scrawled yet more ancient symbols. Are they clues? Or threats?

When the same symbols appear on Maura Isles' door, Maura and Jane must call on the Mephisto Club for assistance. Because this is a form of evil Boston PD has never encountered before. And the only way they can defeat it is by turning to the people who understand the devil himself.