This is the basic idea:
A detective (sometimes 2) go to a charming British countryside manor. An old man or perhaps old woman is going to change or has already changed their will. Their children, servants, etc are angry about it. During the night, the old manlady dies. Natural causes are suspected. But then a clue appears that makes it almost certainly murder! The detectives talk to everyone, and finally the most unlikely person is proved to be the killer. Disguises played a key part in the intrigue. Everyone is shocked, save for the detectives, who knew it all along.
Anyone who has read Agatha Christie can agree that the above synopsis would describe a LOT of her books. I must confess, however, that there is something so charming? enjoyable? about post-WWI Britain that keeps drawing me back. I don't care how formulaic the books are. I like them. I still almost never guess who did it, or how exactly they carried it out. And no article on Agatha Christie novels would be complete without a tip-of-the-hat to her most surprising, out-of-the-box novels, And Then There Were None and The Murder of Roger Ackroyd. ATTWN gives me the creeps every time I read it. I think I was probably around 10 years old when I read it the first time, and it scared me so badly I couldn't sleep. I checked it out from the library last week and made sure I read it outside in the daytime, while Sam played in the sandbox. TMORA has outraged mystery readers for decades. I love it. I don't care if she cheated. The mystery world needs the occasional upset to stay fresh.
I have to thank my grandma Lauver for my love of mystery novels. Her collection of mystery books is one of my favorite things about her house. She introduced me to Dame Agatha and the Cat Who mystery series when I was pretty young and desperate for something challenging to read. I think she also influenced my choice of future novels by giving me series that aren't violent and don't have a lot of foul language or sex. Thanks Grandma, for giving me the good stuff to read.