Mary Debenham and Colonel Arbuthnot were in love.

Mary Debenham and Colonel Arbuthnot are secretly in love, a relationship that develops during their journey from Baghdad and is concealed from the other passengers as part of their involvement in the conspiracy to murder Ratchett. Their romance is a crucial element of the plot, providing a personal motive for Arbuthnot's participation and explaining the intimate, coded exchanges that Poirot overhears. The relationship is revealed only at the end of the novel, when Linda Arden, Mrs. Hubbard, confirms that the two are in love, and Poirot acknowledges their mutual affection.

Identity and Background

Mary Debenham is a young Englishwoman who served as a governess in the Armstrong household in New York, where she was present during the kidnapping and murder of little Daisy Armstrong. Colonel Arbuthnot is a British Army officer, a friend and former comrade-in-arms of Colonel Armstrong, having served with him in the war. Arbuthnot was present in New York during the tragic aftermath of the kidnapping, and he participated in the initial planning of the revenge against Cassetti. Their shared connection to the Armstrong family and their grief over the tragedy form the foundation of their bond.

Development of the Relationship

The relationship between Mary Debenham and Colonel Arbuthnot develops during their journey from Baghdad, where they first met while sharing a railway convoy car from Kirkuk to Nissibin. They travel together on the Taurus Express, and Poirot observes their interactions, noting that Arbuthnot is "susceptible" and that they share a "rather an odd little comedy." At Konya station, Poirot overhears a tense, whispered exchange between them: Mary says, "Not now. Not now. When it's all over. When it's behind us—then—" This phrase, which Poirot later recalls, indicates that their relationship is contingent upon the completion of their secret mission—the murder of Ratchett. Arbuthnot's concern for her is evident when he expresses his dislike of her being a governess, and she responds with a laugh that hints at uncontrol.

Concealment and Deception

Throughout the investigation, both Mary and Arbuthnot maintain a facade of being mere acquaintances who met on the train. Arbuthnot, when questioned by Poirot, insists that Miss Debenham is "a lady" and that she could not possibly be implicated in the crime. He refuses to explain the meaning of the overheard words, stating that his lips are sealed. Mary, when confronted, initially lies about her connection to the Armstrong family, claiming she has never been in America, in order to protect her employment prospects. She later admits the lie but refuses to explain the overheard conversation, saying it concerned "a task I had undertaken." Their coordinated deception is part of the larger conspiracy, designed to prevent suspicion from falling on any individual.

Revelation and Confirmation

The truth of their relationship is finally revealed by Linda Arden, Mrs. Hubbard, in her confession. She states, "Mary and Colonel Arbuthnot—they love each other," and pleads with Poirot to spare them from prosecution. Poirot, in his final solution, acknowledges this, noting that Arbuthnot's presence on the train is explained by his friendship with Colonel Armstrong and his love for Mary. The relationship is thus confirmed as a genuine, deep affection that has developed amidst the shared trauma and the planning of the murder.

Thematic Significance

The romance between Mary Debenham and Colonel Arbuthnot serves as a counterpoint to the novel's themes of justice, revenge, and collective guilt. Their love represents a personal, hopeful future that stands in stark contrast to the violent, communal act of retribution. It also underscores the human cost of the Armstrong tragedy, showing how the lives of those connected to the family were irrevocably altered, and how new bonds of love and loyalty were forged in the aftermath of loss. Their relationship, kept secret until the end, is a testament to the power of personal connection in the face of overwhelming darkness.