Una rica heredera casada con un noble arruinado decide divorciarse y marcharse a Cannes, pero es asesinada en su compartimiento de un lujoso tren. Por desgracia para el marido, alguien lo ha visto en el vagón en el que viajaba su esposa. El misterio del tren azul revela una sutil trama criminal que, como siempre, será revelada por Hércules Poirot.
"For God's sake, come!" Unfortunately, by the time Hercule Poirot received Monsieur Renauld's urgent plea, the millionaire was already dead, stabbed in the back, lying in a freshly dug grave on the golf course of his adjoining Merlinville estate. There's no lack of suspects: his wife, whose dagger served as the weapon; his embittered son, who would have killed for independence; and his mistress, who refused to be ignored; and each felt deserving of the dead man's fortune. The police think they've found the culprit. Poirot has his doubts. A second murder proves him right.