Aboard the luxurious Blue Train bound from Calais to the French Riviera, an unhappy American heiress travels toward a reunion she has long awaited. By the time the carriages reach Nice she lies dead in her compartment, strangled, and the famous ruby she carried against her father's warning has vanished. Hercule Poirot, taking the same train south for a quiet holiday, finds himself drawn into the case almost at once. The dead woman left behind a tangle of relationships any of which might conceal a motive: a millionaire father determined to see justice done, an estranged husband burdened by debt and a bad reputation, a former lover whose aristocratic title may be as false as his charm, and a jealous dancer with secrets of her own. A reserved Englishwoman who has recently come into money and shared a few words with the victim becomes Poirot's unlikely confidante as he reconstructs the final hours. Working backward from a missing jewel and a glimpsed stranger, Poirot weighs each account against the others, testing alibis and watching for the small inconsistency that betrays a careful liar. The pleasures of wealth, travel, and reinvention run alongside older grudges and the lure of a priceless stone, and Poirot's task is to decide which of them turned to murder. Drawing on patience and his understanding of human nature, he sifts performance from truth among people who all have something to hide.