Mayerling

In the short story The Unexpected Guest (later, chapter 1 and 2 of The Big Four), Mayerling was a British secret service agent. According to Inspector Japp, he went missing five years before the events in the story while on a mission somewhere in Russia. At the beginning of the story, Mayerling broke into Poirot's bedroom. He was thin and emaciated and caked in dust and mud. He appeared to be in a catatonic state and would say nothing except "M. Hercule Poirot, 14 Farraway Street." Given a piece of paper by Dr Ridgeway, Mayerling scrawled a dozen figures of the digit "4". When Hastings mentioned “The Big Four”, Mayerling spoke in a telegraphic voice as though reciting the contents of a report. He gave Poirot the first information about the criminal organisation known as "The Big Four" and described briefly who the four key figures involved were.

Mayerling was later found murdered in Poirot's rooms, having been forced to inhale Prussic acid. An attendant arrived from Hanwell asylum and said Mayerling had been an inmate for the last two years and had just escape. He had a persecution mania and kept talking about Chinese secret societies which had locked him up. This attendant was later found to be an imposter and Hanwell denied that any inmate had escaped.