Okay, I've seen a few people ask about the plot for 'The Long Way Home', and I think it's actually one of the more interesting setups in that series.
It starts with Peter and Bunter getting that odd letter from Charles Parker, which hints that Lord Peter might be in some trouble down in France. Harriet Vane is understandably agitated, but she has this quiet, determined resolve that I really admired. She doesn't just sit around; she starts digging, using her novelist's eye for detail and character to piece together where he might have gone.
The core of it is a kind of reverse mystery. Usually, Lord Peter is the one solving the crime. Here, he is the mystery. Harriet's journey through the French countryside, retracing steps he might have taken, talking to people who might have seen him, it turns into this deeply personal quest. It's less about a villain with a plan and more about the aftermath of war, trauma, and finding someone who might not want to be found. The resolution hinges on memory and identity in a way that felt surprisingly modern for its time.