London, 1665. A serial killer stalks his prey, scalpel in his hand and God's vengeance in his heart.      Five years after his restoration to the throne, Charles II leads his citizens by example, enjoying every excess. Londoners have slipped the shackles of puritanism and now flock to the cockpits, brothels and, especially, the theatres, where for the first time women are allowed to perform alongside the men.      But not everyone is swept up in the excitement. Some see this liberated age as the new Babylon, and murder victims pile up in the streets, making no distinction in class between a royalist member of parliament and a Cheapside whore. But they have a few things in common: the victims are found with gemstones in their mouths. And they have not just been murdered; they've been . . . sacrificed.    Now the plague is returning to the city with full force, attacking indiscriminately . . . and murder has found a new friend.