 An elderly amateur criminologist finally figures out how a five-year-old "locked-room" murder was committed, and is murdered by the killer before he can alert Inspector Queen. Ellery takes up the challenge himself. The criminologist had first implicated an airline pilot, because he was flying the plane where the murder was committed and had the opportunity to toss the murder weapon out the window. Nobody else seems to have been able to get rid of the weapon, but Ellery finds that the murder victim had stolen someone else s invention, and that the murderer may have invented the weapon as well.