4 ideas
Members of a university mystery club holiday on a remote Japanese island with a fellow student whose late grandfather scattered moai statues across it as a coded puzzle pointing to hidden diamonds. When a storm cuts the island off from outside contact, two people are shot dead in a locked room. The club's senior member, Jirō Egami, solves both the treasure riddle and the murders through pure deduction from clues the reader has also seen.
Before the solution, the narrative stops and formally declares that every clue needed to identify the culprit has now been shown. This turns the story into a verifiable logic problem: the ending must follow from disclosed evidence alone, and the author can be held to that promise. It rules out solutions that rest on hidden information, sudden confessions, or intuition.
A storm that severs all contact with the outside world does more than create suspense. It fixes a finite, known list of people who could have acted, so the detective can reason by exhaustive elimination. Each deduced constraint, such as who could have handled the weapon or reached the room, strikes names from the list until one remains.
The narrator is a student character who shares the author's name. He reports events honestly but misreads them, while a separate detective figure interprets them correctly. Because the narrator sees exactly what the reader sees, the reader gets the full evidence without the correct inference. The gap between the facts recorded and the conclusions drawn becomes the space where the reader competes.