One day, a man comes across a detective novel in a used book store, suddenly remembers that he was one of the first to discover a murder in a supposedly closed room at a mansion more than 10 years ago, and learns that the culprit has not been caught yet.

One day, a man comes across a detective novel in a used book store, suddenly remembers that he was one of the first to discover a murder in a supposedly closed room at a mansion more than 10 years ago, and learns that the culprit has not been caught yet.