The story is typical country house mystery, with various guests assembled for a weekend stay when one of them is unexpectedly bumped off. The victim in this case is Sir Arthur Billington-Smith, a cruel cold-hearted officer who is universally disliked, and the grisly murder is achieved by means of a dagger in the back of the neck.
Georgette Heyer is very attentive to all of her characters, and in all of her books she skillfully manages to breathe life into even the most insignificant of them. The country house mystery is the perfect genre for such an author; being an ensemble piece where each character needs to have a distinctive personality. The Unfinished Clue brings us a weak and fluttery wife/widow, a practical and sensible sister-in-law, an ineffectual son, a snide nephew and a redoubtable old lady, to name just a few.
The victim's son is accompanied by a flamboyant new fiancee from Mexico by the name of Lola, whose outrageous comments and unusual fashion sense provide much amusement and ensure the reader doesn't take things too seriously. The detective who is called in from Scotland Yard to solve the mystery is very sympathetic, and I was glad to see that the author didn't abandon her romantic tendencies completely while writing this mystery.
Unusually for an Agatha Chistiesque tale like this, there wasn't a lot of focus on objects as clues, with a couple of notable exceptions. There was none of the "small piece of lint, a green feather, a poker placed on the right side of the hearth and a footprint on the ceiling" kind of detective work which I often find so confusing in mysteries of this kind. That doesn't mean I guessed the identity of the murderer here though, the twist in this tale had me completely fooled and I was left smiling ruefully to myself when the murderer was finally revealed.
I think I enjoyed this the most out of all the mysteries I've read for the challenge so far and, though I'll always love her romances the best, I'd definitely be happy to read another of Georgette Heyer's detective stories again.