Masterman was the servant and personal attendant to Colonel Armstrong.

Edward Henry Masterman, the pale, inexpressive English valet of Samuel Edward Ratchett, was not merely a servant hired nine months earlier but a man with a deep, concealed tie to the Armstrong tragedy: he had served Colonel Armstrong as his batman during the war and later as his personal valet in New York. This prior loyalty, which he initially denied during the investigation, placed him among the twelve conspirators who had condemned Ratchett—the man who had destroyed the Armstrong family—to death.

Identity and Background

Masterman was thirty-nine years old and gave his home address as 21 Friar Street, Clerkenwell. He had previously been employed by Sir Henry Tomlinson in Grosvenor Square, leaving only when Sir Henry departed for East Africa. His manner was correct and unemotional, his face inexpressive, and he spoke with the clipped reserve of a well-trained English servant. He had been with Ratchett for just over nine months, having been hired after the previous secretary left. During the journey, he shared a second-class compartment (No. 4) with the Italian Antonio Foscarelli, a man whose voluble nature contrasted sharply with Masterman's own reticence. He occupied himself by reading Love's Captive by Mrs. Arabella Richardson, a novel he found highly enjoyable.

Concealed Connection to the Armstrong Case

When Poirot first questioned him, Masterman denied any knowledge of the Armstrong kidnapping case beyond what he had read in the papers, calling it "a very shocking affair." He expressed disbelief when told that Ratchett was the principal instigator, saying "I can hardly believe it, sir." However, after the investigation had progressed and Poirot had begun to uncover the conspiracy, Masterman came forward voluntarily to the dining-car. He admitted that he had been Colonel Armstrong's batman in the war and later his valet in New York, and that he had concealed this fact during the morning's interview. He did so, he explained, to protect Antonio Foscarelli, the Italian chauffeur whom he swore "wouldn't hurt a fly" and who, he could positively attest, never left their compartment all night.

Role in the Conspiracy and the Murder

Masterman's role in the plot was essential. As Ratchett's valet, he had daily access to his master and could administer the sleeping draught without suspicion. He confirmed that Ratchett always took a sleeping draught when travelling by train, and that on the night of the murder he had poured it into a glass and placed it on the toilet table. He also admitted that Ratchett had been upset over a letter he had found in his compartment, which Masterman denied having placed there. This letter was almost certainly the final warning from the conspirators. Masterman's alibi for the time of the murder was provided by Foscarelli, who swore that the valet never left the compartment all night, a claim Masterman himself corroborated by stating he had the toothache and lay awake groaning until about four in the morning. This alibi, like all the others in the case, was a carefully constructed lie. He was one of the twelve who entered Ratchett's darkened compartment through Mrs. Hubbard's and struck with the dagger, each blow contributing to the collective execution.

Narrative Significance

Masterman's quiet, unassuming demeanor made him an ideal conspirator. His initial lies and his eventual, reluctant confession exemplified the pattern of deception that Poirot had to unravel. His presence on the train was not a coincidence but a deliberate part of the plan, ensuring that Ratchett's own household was infiltrated by those who had sworn to see him dead. His testimony, when it finally came, served not to incriminate himself but to protect his fellow conspirator, Foscarelli, demonstrating the deep loyalty that bound the twelve together.