Poirot investigates the murder of Florence Carrington while traveling on the express train to Plymouth. Her father, mining entrepreneur Gordon Halliday, will spare no expense to have the crime solved. She had recently been approached by her estranged husband Rupert, asking for money and was seen having lunch with a one-time suitor, Armand de la Rochefort, of whom her father disapproves. However, the victim's jewelry was stolen and Poirot realizes that to find the murderer, they must first find the jewels.