Summary: After the war, Lord Peter starts telling Harriet about his first case, and that telling leads to a new case.
One of the ongoing themes of this set of four books is that there is a play between what is real as a mystery and how mysteries can be written about to be believable. As a setup for the rest of the book, Lord Peter describes to Harriet his first case and how he started as a detective. Throughout this early section, Harriet and Lord Peter talk about whether something would be believable if it were in one of Harriet’s novels vs in one of Peter’s cases. This is a running gag in the series because Peter is often asking Harriet what he should do or if she were writing the story what the perpetrator would do at that point. It is both a running gag, but also a serious discussion about the nature of reality and how the nature of writing works. You can’t just write a story, you have to fall within the set of conventions that seem believable unless you are intentionally subverting the conventions to suggest that the conventions themselves are not believable.
Part of the thread of the book is that the Attenbury Emeralds, which is what his initial case was about, has continued to come up again and again over the years. That is improbable, but it is improbable because there is more to the story than what it initially seems.
There are definitely different types of thriller/mystery stories. Some stories invite the reader to figure out what the story is as the clues are dropped. Some stories do not really give clues as much as narrate the story so that the mystery is slowly revealed. And some stories are thrillers where the point is the thrill, not the mystery. (And there are other types as well.) The reader doesn’t know what the reader doesn’t know, so as Walsh is playing with the conventions here, the improbable becomes the only option as time goes on.
Personally, I tend to like mystery series more for the character development than the specific mystery. The end of the book brings about something that was hinted at in the previous book and is more fully developed in the fourth book of the series.