The Mysterious Mr. Quin by Agatha Christie

"Mr. Satterthwaite was sixty two-a little bent, dried up man with a peering face oddly elflike, and an intense and inordinate interest in other people lives. All his life, so to speak, he had sat in the front row of the stalls watching various dramas of human nature unfold before him. His role had always been that of the onlooker. Only now, with old age holding him in its clutch, he found himself increasingly critical to the drama submitted to him. He demanded now something a little out of the common."

Mr. Satterthwaite was a guest of the New Year's Eve house party at Royston. The elder members of the party assembled in the big hall and young people had gone to bed. Then he had a sensation that something interesting was happening or going to happen.

That was a wild night.

"The wind rose in another terrific wail, and as it died away there came three loud knocks on the big nailed doorway.

Framed in the doorway stood a man's figure, tall and slender."

The stranger named Mr. Harley Quin was no ordinary guest. He had come to unmask a murderer.

With the help of Mr. Quin, Mr. Satterthwaite solved many unsolved problem thus participating in the dramas of human nature.