The Mystery of the Blue Train
Agatha Christie
 Poirot boards Le Train Bleu, bound for the French Riviera. So does Katherine Grey, who is having her first winter out of England, after having inherited a huge sum. While on board she meets Ruth Kettering, an American heiress bailing out from a marriage to meet her lover. The next morning, though, Ruth is found dead in her compartment, a victim of strangulation. The famous ruby, "Heart of Fire", which had recently been given to Ruth by her father, is discovered to be missing. Ruth's father, the American millionaire Rufus Van Aldin, and his secretary, Major Knighton, convince Poirot to take on the case. Ruth's maid, Ada Mason, says she saw a man in Ruth's compartment but could not see who he was. The police suspect that Ruth's lover, the Comte de la Roche, killed her and stole the rubies, but Poirot does not think he is guilty. He is suspicious of Ruth's husband, Derek Kettering, who was on the same train but claims not to have seen Ruth. Katherine says she saw Derek enter Ruth's compartment. This also throws suspicion on Derek when a cigarette case with the letter K on it is found. Poirot investigates and finds out that the murder and the jewel theft might not be connected, as the famous jewel thief The Marquis is connected to the crime. Eventually, the dancer Mirelle, who was on the train with Derek, tells Poirot she saw Derek leave Ruth's compartment around the time the murder would have taken place. Derek is then arrested. Everyone is convinced the case is solved, but Poirot is not sure. He does more investigating and learns more information, talking to his friends and to Katherine, eventually coming to the truth. He asks Van Aldin and Knighton to come with him on the Blue Train to recreate the murder. He tells them that Ada Mason is really Kitty Kidd, a renowned male impersonator and actress. Katherine saw what she thought was a boy getting off the train, but it was really Mason. Poirot realized that Mason was the only person who saw anyone with Ruth in the compartment, so this could have been a lie. He reveals that the murderer and Mason's accomplice is Knighton, who is really The Marquis. He also says that the cigarette case with the K on it does not stand for Kettering, but Knighton. Since Knighton was supposedly in Paris, no one would have suspected him. Derek did go into the compartment to talk to Ruth once he saw she was on the train, but he left when he saw she was asleep. The police then arrest Knighton, and Van Aldin thanks Poirot for solving the case.