The girls, who were not obstacles exactly, but line items — two people who would one day inherit a share of something and who currently occupied the emotional real estate she needed.

And Rosamund, who was in the way, and who was the easiest to remove, because she was staff, and staff can be replaced by a phone call.

The engagement was announced formally in early November.

Within three weeks, the temperature inside that house began to change again — and this time, the only adults who noticed were a housekeeper and a nine-year-old girl.

Part Four: The Poison, Administered in Small Doses