Poirot discovers that Madame Giselle was known for blackmailing her clients who hadn't paid up. Also, Madame Giselle had an estranged daughter who should inherit her mother's estate; she might be on board the plane. Poirot questions several of the passengers including Mr. Clancy, a detective novelist. Countess Horbury also comes under suspicion. She came from the lower class but married well. In the meantime, her husband has cut her off and she had owed Madame Giselle money. The Countess' maid, called into the compartment during the flight, would have had the perfect opportunity to commit the crime. The maid is revealed as the long lost daughter of the victim. It appears as if she is guilty, but she in turn is murdered on the boat train to Bologne.
Dentist Norman Gale, who had a crush on the novel's heroine Jane Grey, is revealed to be Anne's new husband. Poirot discovers that Gale brought his dentist's jacket on board and excused himself to go to the washroom. He donned the jacket and posed as a steward. Under the premise of delivering a coffee spoon to Madame Giselle, he stabbed her with the poison dart. Gale's intention had been to frame the Countess. The blowpipe found behind Poirot's seat was supposed to be behind the Countess' but they had switched at the last minute. Poirot allows the detective novelist to listen in as he joins the dots in the story's denouement.