The story begins when Katherine Grey, a young woman recently returned from a period of service as a companion to a wealthy woman, is traveling aboard the Blue Train to meet her friend Lady Tamplin in Nice. Katherine’s journey takes an unexpected turn when she is caught up in a dramatic murder mystery.

During the journey, a woman named Ruth Kettering, the wife of a wealthy American businessman, is found murdered in her compartment. Ruth had been traveling with her husband, but her death appears to have been part of a larger scheme involving jewels. Ruth’s precious necklace, a magnificent and valuable set of jewels, is missing after the murder.