4 ideas
After blind New York art dealer Georg Khalkis dies, his will vanishes from his wall safe during the funeral. The young Ellery Queen has the coffin exhumed and finds a second body inside, the strangled ex-convict Albert Grimshaw. Ellery confidently announces a solution built on clues the killer had planted, watches it collapse, and finally identifies the murderer as someone working inside the official investigation, who had been positioned to steer it all along.
Just before the solution, the author stops the narrative and declares that the reader now holds every fact needed to name the culprit by logic alone. This turns the novel into a verifiable puzzle. The author must plant every necessary clue openly, and the ending must be deducible from them, not revealed by new information or luck.
A culprit who knows the investigator reasons strictly from physical evidence can manufacture clues that lead step by step to a chosen wrong answer. The more predictable and logical the method, the more precisely it can be hijacked. The defense is to ask who could have anticipated what the detective would infer from a clue, and to treat a too-convenient chain of evidence as possibly authored.
Deduce a list of attributes the criminal must have had, such as access at a specific moment, knowledge of a particular fact, and the opportunity to plant or remove a specific clue. Then test every suspect against all of these conditions at once. The solution is whoever satisfies every condition, which makes the answer a matter of elimination rather than intuition about motive.