"Trust the train, Mademoiselle, for it is le bon Dieu who drives it. [...] And trust Hercule Poirot. He knows."

Unhappy with her marriage, Ruth Kettering, a pampered American heiress, is on her way to meet her lover in the French Riviera, but she will never reach her destination. When the train stops in Nice, the heiress is found dead and without her precious jewels aboard Le Train Bleu. But in the midst of robbery, murder, and tragedy, famed detective Hercule Poirot is there to solve the mystery.

The gruesome murder is believed to have been performed by Ruth's estranged husband, but Poirot believes there is more to the story. On top of the motive of murder, Poirot must consider the theft of the famous ruby, "Heart of Fire," that was given to Ruth by her father, the American millionaire Rufus Van Aldin. And it is her father's secretary, Major Knighton, who convinces Poirot to unmask the perpetrator of the crime. Poirot receives conflicting stories from the key players, witnesses, and suspects aboard the train, and so to find the murderer, the Belgian detective must stage a reenactment of the fatal journey.