The Mystery of the Blue Train

Poirot boards Le Train Bleu, bound for the French Riviera. So does Katherine Grey, who is having her first winter out of England, after recently receiving a relatively large inheritance. On board the train Grey meets Ruth Kettering, an American heiress leaving her unhappy 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, American millionaire Rufus Van Aldin, and his secretary, Major Knighton, persuade Poirot to take on the case. Ruth's maid, Ada Mason, says that 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 ruby, but Poirot does not think that the Comte is guilty. He is suspicious of Ruth's estranged husband, Derek Kettering, who was on the same train but claims not to have seen Ruth. Katherine says that she saw Derek enter Ruth's compartment. Further suspicion is thrown on Derek when a cigarette case with the letter "K" is found there.

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 avaricious Mirelle, who was on the train with Derek—with whom she had been having an affair but, now spurned, is seeking revenge against him—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 realised that Mason was the only person claiming to have seen 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 ruthless "Marquis". He also says that the cigarette case with the K on it does not stand for 'Kettering', but for '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 arrest Knighton and the case is closed.