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 claims 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. Characters Hercule Poirot, a private detective Rufus Van Aldin, the American millionaire, Ruth's father Ruth Kettering, Van Aldin's only daughter, Derek's wife Hon Derek Kettering, Van Aldin's son-in-law, Ruth's husband Mirelle, a Parisian dancer, Derek's gold-digging and later vengeful French lover Major Richard Knighton, Van Aldin's secretary Ada Mason, Ruth Kettering's maid Armand, Comte de la Roche, former lover of Ruth Monsieur Carrège, of French police Commissary Caux, of French police Mr Goby, Rufus's informant Katherine Grey, formerly companion to the recently deceased Mrs Harfield, who has left her entire estate to Grey Dr and Mrs Harrison, Katherine's friends in the village of St Mary Mead Amelia Viner, an elderly, terminally ill spinster and friend of Katherine Grey in St Mary Mead Rosalie, Viscountess Tamplin, cousin of Katherine, owner of a villa on the Riviera Hon. Lenox Tamplin, Lady Tamplin's daughter Charles Evans, Lady Tamplin's much younger husband Kyrios (Mr) Papopolous, jewellery dealer and an acquaintance of Poirot Zia Papopolous, his daughter Pierre Michel, the train's attendant Joseph Aarons, Poirot's acquaintance, an expert in people involved in the "dramatic profession" (acting) who advises Poirot about the skilled impersonator Kitty Kidd, known by a different name for most of the novel Influence and significance The novel's plot is based on the 1923 Poirot short story "The Plymouth Express" (later collected in book form in the US in 1951 in The Under Dog and Other Stories and in the UK in 1974 in Poirot's