It's been quite a while since I read an Agatha Christie mystery. I prefer Miss Marple to Hercule Poirot, so Murder with Mirrors was a good choice.
An old school friend, Ruth Van Rydock, asks Miss Marple to go check on her sister, Carrie Louise, also an old school friend of Miss Marple's. Ruth had visited recently and felt that something was wrong. She couldn't put her finger on it, but she thought Miss Marple could. She wasn't wrong.
Carrie Louise and her husband run a school for wayward boys at their country house. Carrie Louise's husband, Lewis Serracold, was sure that the boys, headed for a life of crime, could be persuaded to pursue legitimate careers. The school provided training and psychological counseling.
Shortly after Miss Marple arrives, Carrie Louise's step-son is shot to death in the house while most of the household is listening to a fight between Lewis and one of the students. Shots are fired, they open the door, and Lewis and the boy are fine. Then they discover that Christian Gulbrandsen has been shot to death in another room.
The house is full of family: a grown daughter, a granddaughter (daughter of an adopted daughter), a step-son, and two sons of a former husband of Carrie's. I had trouble keeping the relationships straight.
Carrie Louise never thinks anyone is bad. She lives in a Pollyanna world. Or does she? Maybe she sees more clearly than other people. Everyone loves her, she's sweet and kind. So who's trying to poison her?
In classic Christie style, there are plenty of red herrings, lots of suspects, many diversions.