In which Sayers takes another stab at marrying off Lord Peter and being done with him. That part goes awry. However, she does produce a marvelous mystery in the process.
Harriet Vane is off on a walking tour through the country. Her picnic on one beach culminates in finding a dead body, with a conspicuously cut throat. And then she has a horrible time getting to official attention, by which time the corpse is long lost under the tide.
She brazens it out and notifies the press herself. With the media circus beginning, Lord Peter arrives. And they begin looking for clues, of which the first one is that the blood was still red and running when she found the body -- it had to be fresh. But others include a bay horse with a new shoe, volumes of Ruritanian romances, the buying of collars, a double identity, a photograph of a regal looking Russian woman, a farmer who does not bring in hay, and the beard the victim was wearing. Also, many questions about clocks.
Harriet is working on a novel that has a lot of meddling with clocks for its alibi, just as a neat little parallel.
But it all pulls together with her elegant plot.
Point of information with a bit of spoiler about it: [Spoiler (click to open)]He wasn't eligible for the throne anyway. When Catherine the Great died, and her son finally ascended the throne, he laid down a law of succession. (Peter the Great had decreed the Tsar choose his successor. Then he didn't choose his own. It was chaotic like that.) One requirement of succeeding to the throne was that the marriage be equal. A union with someone other than a member of a royal dynasty was automatically morgantic. I'm not sure about female line descent either. To be sure, Russia was semi-Salic -- a woman could ascend the throne -- but the condition was the extinction of all male dynasts.
Which is not relevant to the plot in the end.