Mary Debenham was the family teacher of Armstrong.

Mary Debenham, a young Englishwoman traveling from Baghdad to London on the Orient Express, was secretly the governess in the Armstrong household during the infamous kidnapping and murder of three-year-old Daisy Armstrong. This connection placed her at the heart of the tragedy that destroyed the Armstrong family and ultimately motivated the elaborate conspiracy to execute Samuel Ratchett, the man responsible for the crime. Her deliberate concealment of this past, and her emotional breakdown when confronted, became a crucial thread in Poirot's unraveling of the case.

Identity and Background

Mary Debenham was employed as a governess in the Armstrong household in New York, where she taught the younger sister of Sonia Armstrong, Helena Goldenberg (later Countess Andrenyi). She was present in the house when Daisy Armstrong was kidnapped and killed, and she witnessed the subsequent collapse of the family: Sonia Armstrong died in childbirth after the shock, Colonel Armstrong committed suicide, and the French nursemaid Susanne, wrongly suspected of complicity, threw herself from a window. The trauma of these events left a deep mark on Mary, who later told Poirot that she had "learned to save myself useless emotion." After the tragedy, she left America and rebuilt her life as a governess in the Middle East, carefully hiding her connection to the case to protect her employment prospects.

Concealment and Confrontation

When Poirot first interviewed Mary on the snowbound train, she denied ever having been in America and claimed she had never seen Ratchett before the journey. She also refused to explain the cryptic words Poirot had overheard at Konya station—"Not now. Not now. When it's all over. When it's behind us"—spoken to Colonel Arbuthnot. Poirot, suspicious of her composure and her evasions, pressed her further. In a second interview, he revealed that he knew she had been in the Armstrong household. Mary admitted the lie, explaining that she feared the publicity of being connected to a murder case would ruin her career: "I have my living to get." When Poirot asked her to identify Countess Andrenyi as Helena Goldenberg, Mary broke down, crying uncontrollably, and fled the dining car.

Role in the Conspiracy

Mary Debenham was one of the twelve conspirators who planned and carried out Ratchett's murder. According to Linda Arden (Mrs. Hubbard), Mary "worked out all the details later with Hector MacQueen." She was responsible for the meticulous planning of the alibis and the staging of evidence. Her compartment was shared with Greta Ohlsson, the Swedish nurse who had cared for Daisy, and her alibi for the time of the murder was provided by Ohlsson. Her relationship with Colonel Arbuthnot, a friend of Colonel Armstrong, was genuine—they were in love—and their pretense of being strangers was part of the conspiracy to avoid suspicion. Poirot noted that her "cool, intelligent, resourceful brain" made her a natural architect of the plot.

Thematic Significance

Mary Debenham embodies the theme of justice versus the law. Her participation in the murder was not an act of personal vengeance but a calculated, collective decision to carry out a sentence that the legal system had failed to deliver. Her initial lies and her emotional fragility when exposed reveal the psychological cost of the conspiracy: she was not a cold-blooded killer but a woman driven by loyalty to a destroyed family and a desire to prevent future crimes. Poirot's final solution, accepted by M. Bouc and Dr. Constantine, allowed her and the other conspirators to go free, recognizing that the murder was an act of moral justice rather than common criminality.