Yet in the film, Hercule Poirot, sans mustache, seems to be picking up a romance with Salome Otterbourne when the credits roll.
The reason goes back to the opening of the film and the overarching themes throughout this adaptation. At the beginning of the movie, we see the origins of Poirot’s life story, with the future detective serving in the Belgian Army during World War I. There, he is so badly injured by an explosion that his face is permanently disfigured above the lip line. However, his fiancée, a woman named Katherine, is not deterred by the scars and suggests he’ll simply grow a mustache.
As far as I’m aware, this backstory was entirely invented for the film (although actual Christie scholars are encouraged to correct me if wrong!). On the page, a much older Poirot spent the war as a refugee in Britain and was already a police detective before fighting broke out. However on-screen, the film suggests that he became the great detective by 1937, complete with mustache, because of that fateful encouragement to reinvent himself by his fiancée and, as we later learn, her death.
Intriguingly, it is only to Jacquelin, his future prey, whom he confesses this story, revealing that after his injury his bride-to-be was traveling to spend the Christmas of 1916 with him when her train was presumably bombed or derailed in some other manner, killing her. He shut himself off from the world after that and began living in his head. Dining alone.
“You could never understand what people would do for love,” Bouc says in his final words on this earth before being shot in the throat. He was attempting to chastise his friend for insisting on bringing him to justice for stealing (and returning) a necklace, as well as accuse him of being trapped within his cold intellect. How could this cerebral to a fault man, who would betray his own friends over a small larceny, understand that a person could have a momentary lapse in judgment and steal a necklace for love? The theft, though unforgivable, would allow him to remain wealthy while marrying Rosalie without his mother’s blessing.
And yet, the overarching theme is everyone does irrational things for love. Jackie planned the murder of her best friend, whom she apparently did like to a point, in order to satiate the greed of the man she loved; Jackie’s deception of stalking Simon and Linnet may have been feigned but also spoke of love’s obsession; and Bouc became a criminal so he could be with Rosalie, burden-free and debts paid. Even Poirot himself became the man we see today, this lonely genius, because he lost his great love 20 years earlier and has since hidden behind her suggested mustache as if it were an armored mask.