While staying at the Park Lane hotel, Poirot meets Rufus Van Aldin and his daughter, Ruth. Van Aldin, an American, is hugely wealthy (through Oil) and there is much gossip surrounding his name after his wife, a dancer, mysteriously disappeared after Ruth's birth, reportedly back home to Buenos Aires after ruining herself through drink. Rumour has it that Van Aldin had her 'bumped off'. Ruth invites Poirot to join in her birthday celebrations at the hotel that night. There she shows off her birthday gift from her father, a magnificent jewel called the Heart of Fire which has a notorious history including murder and betrayal, and as such is described as 'sharper than diamond, redder than blood". It is during the party that we first meet Ruth's husband, Derek Kettering. It becomes clear that their marriage is not made in heaven. He has a gambling problem and is truly disliked by Rufus Van Aldin. That night Van Aldin confronts Kettering and offers him £100,000 to agree to the divorce that Ruth will serve him the following morning. Kettering refuses. The same night Poirot befriends a young heiress, Katherine Grey. Katherine is struggling to acquaint herself with the etiquette of her new surroundings and Poirot comes to her rescue. She tells him she is taking the Blue Train to Nice as she has been invited to visit the Villa Marguerite by her cousin, Lady Tamplin. Little does she know that Lady Tamplin is broke and there is an ulterior motive behind the invitation. Poirot is enchanted by Katherine and is delighted when she agrees to his being her companion on the Blue Train. The following day, anyone who is anybody boards the Blue Train in Calais. Included are Ruth and Derek Kettering; Ada Mason (Ruth's maid); Poirot and Katherine; Mirelle Milesi (Van Aldin's jealous mistress); and Ruth's lover, the Comte La Roche. As the train is about to depart, Lady Tamplin rushes to board along with her very young 4th husband, Corky, and daughter, Lenox. It is apparent they live a very Bohemian lifestyle. Once underway, Ruth approaches Katherine and asks if she would exchange compartments with her so that she can be closer to the Comte la Roche who is travelling in 3rd Class. Katherine agrees, but is then evidently thrown when she hears Ruth Van Aldin's name. She explains to Poirot the reason why. Reluctantly, her father agreed to sell his business to Van Aldin Oil on the understanding that they retained the entire workforce, but within a week they had sacked everybody. Her father could not live with the betrayal and killed himself. CONT .../ Katherine excuses herself from dinner. As Poirot leaves her compartment, he sees the Steward talking to Miss Van Aldin and telling her he will order a dinner tray for her to have in her compartment. In the early hours the next morning, the Blue Train arrives at Marseille. It is there that Poirot hears the smashing of glass and sees a figure hastening down the corridor. On looking out of the window, he sees Corky running to board the train again. He is puzzled. At last the train reaches its destination, Nice. Poirot climbs on to the platform and hears the piercing scream of Lady Tamplin. It appears Katherine has been murdered. Poirot is hugely relieved when Katherine emerges from her compartment - he realises immediately it must be Ruth Kettering whose body has been found - but she has been so badly beaten around the head with a hammer that there is a question mark over her identity. Poirot looks around the crime scene. There is no sign of the Heart of Fire stone, but the strongbox doesn't appear to be forced. Poirot notices a hair which he traps with tweezers, followed by the last page of a letter from Sister Rosalia at an asylum in Nice, and finally a cigarette lighter. As Ruth's body is taken away from the station, Rufus Van Aldin appears. He tells Poirot that he left his secretary, Major Knighton, at the hotel in Paris and took a plane to Nice in order to surprise his daughter. Together with Inspector Caux, Poirot questions all those on the train. It appears that Kettering and La Roche were playing cards with Corky in 3rd Class all night, therefore providing themselves with alibis. Lady Tamplin persuades everyone back to the Villa Marguerite. While a party is in full swing, Poirot searches the Tamplin's bedroom and discovers what Corky gave Lady Tamplin for an anniversary present. That night, while Katherine is sleeping, an intruder enters the bedroom with a knife and attempts to murder her, but is forced to flee after Lenox jumps on his back and bites his neck. Poirot now has to accept that perhaps the murder on the Blue Train was a case of mistaken identity - perhaps Katherine had been the intended victim all along. Slowly Poirot manages to unravel the case. La Comte proved to be a con man and intended to steal the Heart of Fire stone by replacing it with a replica. At Marseille he went to Ruth's apartment with a bottle of champagne, only to find her dead. He dropped the bottle - the smashing glass Poirot heard - and hastened back down the corridor to 3rd Class discarding the fake stone out of the window on the way. It was this stone that Corky found and gave to Lady Tamplin as an anniversary present. CONT .../ Poirot summons everyone to the Pullman carriage of the Blue Train to confront the murderer. He tells the story of how it had been Ruth's intention to visit her mother at the Asylum in Nice, and how Van Aldin had travelled to Nice to stop her. How Van Aldin's mistress, Mirelle Milesi, had visited Ruth's compartment (dropping her lighter there) and had discovered the letter about Van Aldin's wife from Sister Rosalia at the Asylum. Van Aldin had asked Mirelle to seduce Kettering on the Blue Train in order to obtain evidence of adultery and thereby push through the divorce. What Van Aldin and Mirelle failed to realise was that Kettering was immune to Mirelle's charms because he was in love with his wife. Poirot then reveals the murder was the work of two people - Ruth's maid, Ada Mason, and her lover Van Aldin's secretary, Major Knighton. Their motive was the theft of the Heart of Fire, but they were aroused by the savagery of murder. Knighton had left the Hotel and boarded the train whilst it was travelling slowly around the suburbs of Paris. Mason helped him gain access to Ruth's compartment where he murdered her. After the murder, Knighton jumped from the train and Mason, dressed in a wig and gown, posed as Ruth to trick everyone into believing that Ruth was alive until the train reached Marseille. This allowed them both to have watertight alibis. Poirot explains that they may have got away for it were it not for one mistake - Knighton fell in love with Katherine, and this was unbearable to Mason who then tried to kill Katherine in a fit of jealously. Poirot shows the bite on her neck. Suddenly Katherine is aware that Knighton is holding a 'knife' to her throat. He drags her down the carriage in hot pursuit by Poirot. Realising he's trapped, Knighton jumps from the train and into the path of an incoming train. Katherine averts her head and looks at what he's left in the palm of her hand - it was not a knife he was holding to her throat, but the Heart of Fire - sharper than diamond, redder than blood.