Since the beginning of history, jewels have exercised a baneful spell. Murder and violence have followed in their wake. So with the famous Heart of Fire ruby. It passes into the possession of the beautiful American woman, Ruth Kettering, and doom fellows swift upon it. Whose hand was it that struck her down ? Were the jewels the motive for the murder, or were they only taken as a blind ? What part did the beautiful foreign dancer play ? These are some of the questions that have to be answered, and the story tells also how these strange and dramatic happenings affect the life of a quiet English girl who has felt convinced that "nothing exciting will ever happen to me. " She uses very nearly those words to a chance acquaintance on the Blue Train - a little man with an egg-shaped head and fierce moustaches whose answer is curious and unexpected. But even Hercule Poirot, for it is he, does not guess how soon he will be called upon to Blue Train steams into Nice the following morning and it is discovered that murder has been done.