The game simulates Agatha Christie style murder mysteries, with the focus being people at a country house in the 1930s. The game is similar to Fiasco in style, giving you the skeleton of structure that you then build on in play to create the details of the world.
You start out making up characters, and your relationships with the other characters. The first half of the game deals with the characters pushing each other to stress the relationships. I can imagine that many details will be invented here that will come back later on, often twisted.
At the halfway point, everyone writes down which character they want to be the murder victim and then a random pick determines who it was. The player of the victim returns in the second half as Inspector Chapel.
The second half of the game plays similarly to the first, with the addition of the ability to investigate people - successful investigation means that something is revealed about the character's relationship with the victim (i.e. giving them motive). Eventually, two people have enough motive to have done the deed and there is a denouement to reveal who the killer really was (determined by the winner of a dice roll - events earlier in the game determine how many dice each player gets).
It seems like a very good structure to make the game work as intended. There's some advice on things to aim for in play, in order to get the right sort of feel.
There's also a lot of information in the book about life in a country house at the time, from an overall history of the evolution of the country house to details like how to address different people and who each of the servants report to.
After that read, it looks very good. I'm keen to play it, and will be looking for opportunities to do so.