Based on the Hercule Poirot short story, The Plymouth Express, published in 1923, this was Agatha Christie’s least favourite story. It was written at the time of her marriage break-up from Archie Christie, and shortly after the death of her mother, Clarissa Miller.

A young woman, Katherine Grey, has recently inherited a large sum of money from the elderly woman to whom she was companion in St Mary Mead. Katherine is therefore taking her first winter abroad, travelling on the luxurious Blue Train to the French Riviera, to stay with her cousin.

On board the Blue Train, Katherine makes friends with Ruth Kettering, an American heiress. Ruth is heading to the South of France to meet her French lover, the Comte de la Roche. Her unhappy marriage to Derek Kettering is soon to end in divorce. Ruth confides in Katherine about her intricate love life.

Ruth Kettering’s millionaire father has recently given her a set of priceless rubies, which includes the famous ruby, The Heart of Fire. Despite his warnings to keep them somewhere safe, Ruth takes the jewels with her on a trip to the South of France.

However the next morning, Ruth Kettering is found strangled in the bed of her train compartment, her face battered beyond recognition, and the Heart of Fire rubies are missing. Hercule Poirot also happens to be travelling on the Blue Train, and Ruth’s father, the American millionaire Rufus Van Aldin, asks Poirot to investigate the murder.

Ruth Kettering’s maid, Ada Mason, tells Hercule Poirot that she saw a man in Ruth’s compartment the night before the murder, but couldn’t see his face, and so is unable to identify him. Rumours abound that the famous jewel thief, the Marquis, is responsible.

Suspicion also falls on Derek Kettering, Ruth’s estranged and adulterous husband, when a cigarette case engraved with the letter K is found in Ruth Kettering’s compartment. Katherine Grey tells Hercule Poirot that she saw Derek enter Ruth’s compartment on the train. However Derek denies this and claims not to have seen Ruth at all, or to have even known that she was on the Blue Train. He had been on the train with his lover, Mirelle, a famous exotic dancer.

Before leaving London, Derek Kettering had been visited by Van Aldin’s secretary, Richard Knighton, who had tried to convince him to accept Van Aldin’s offer of £100,000 to divorce his daughter. He had refused. Now that his wife is dead, Derek Kettering stands to inherit a couple of million pounds.

However the police believe that it is Ruth’s lover, Armand de la Roche, who is responsible for the crime, and they believe that the Comte de la Roche killed her to steal the rubies.

Katherine Grey finds herself entwined in the murder, and helps Hercule Poirot to identify the killer.

The book has all the elements - a journey on a luxury train, an American heiress, jewel thieves, and Hercule Poirot. It is however a strange mix of murder mystery whodunit and an adventure story. Whilst enjoyable, I would agree that it is not one of her best, but by no means her worst!