Captain Humphrey Naylor

In the short story The Under Dog, Captain Humphrey Naylor is the brother of Lily Margrave, the companion of Lady Astwell. On the night the murder of Sir Reuben Astwell, Humphrey was staying at the Mitre Hotel in the nearby village of Abbots Cross. After dinner he had gone out for a walk and returned about half an hour after midnight.

As Poirot's investigations revealed, Humphrey also had a grievance against Sir Reuben. Some time earlier, Humphrey had been in Africa and had discovered the presence of some gold deposits. As exploiting it would have taken a large investment, Humphrey approached Sir Reuben and attempted to interest him. Sir Reuben sent an expert out to the site and allegedly he submitted an unfavourable report. Humphrey subsequently departed on another African expedition and nothing was heard of him for some time.

Later when Humphrey returned, he learnt that a company called Mpala Gold Fields had been formed which appeared to be exploiting the same gold find he had made. Although Sir Reuben did not appear to have any links to this company, Humphrey believed otherwise. He thought that Sir Reuben had deliberately swindled him. His sister Lily Margrave got herself employed by the household of Sir Reuben in order to "spy" on Sir Reuben and discover any evidence of the fraud. While staying at the Mitre Hotel, Humphrey took late night walks after dinner in order to meet his sister to listen to her reports. He met her about two weeks before the events in the story, and once more on the night of Sir Reuben's murder.

Dr Cazalet, who discussed the murder with Poirot, considered that Humphrey was a plausible suspect. His after dinner walk gave him opportunity, and his grievance over the gold mine gave him a motive. The murder weapon was a African club which was one of the artefacts decorating the study. Lady Astwell, under hypnosis, had stated that she had seen or felt someone hiding behind a curtain in her husband's study. This someone could have been Humphrey.

Humphrey was however exonerated by Poirot's revelation of the true killer at the end of the story. Documents discovered by Humphrey's sister Lily proved that Sir Reuben was linked to Mpala Gold Fields and that therefore her brother had been swindled. Whether Humphrey got any redress is not stated.

Portrayals
In ITV's 1993 adaptation of the story, Humphrey Naylor is portrayed by Andrew Seear. In this adaptation, he is not an African explorer but a reseach fellow at Imperial College, specialising in polymer chemistry. He had developed a synthetic rubber called astroprene. As he needed commercial investment to take the innovation forward, he approached Sir Reuben, who in this case, is the owner of Astwell Chemicals. At first Sir Reuben appear interested and then later told Humphrey that they were not prepared to invest. However subsequently, Humphrey began to read hints in academic journals suggesting that Astwell Chemicals had taken his idea and gone ahead to develop astroprene for themselves.

Other than this difference in backstory, his portrayal is fairly similar to that in the original story. He stays at the same golf club as Poirot and Hastings and he did take a late night walk on the night of Sir Reuben's murder to meet Lily. Thus he had opportunity and motive. In this case, Lily was genuinely worried that it might have been Humphrey behind the curtain, so much so that she interfered with the hypnosis of Lady Astwell to prevent her from talking too much.

At the end of the story, Victor Astwell, Sir Reuben's brother and heir, made a settlement with Humphrey about astroprene. The details are not given but Lily told Poirot that the terms were very generous.