Returning from a Christmas shopping trip, Elspeth McGillicuddy glances out of her first-class carriage as another train draws level on a parallel track. Through the window she sees a man strangling a woman, then the trains separate and the moment is gone. No one believes her: the railway staff find nothing, the police turn up no body, and the incident is dismissed as the imagining of a tired traveller. Only her old friend Jane Marple takes the account seriously. Working from timetables, maps and the geometry of the two railway lines, Miss Marple reasons out where a body might have been pushed from the carriage and comes to suspect the grounds of Rutherford Hall, the gloomy estate of the reclusive Crackenthorpe family. Too frail to search the place herself, she recruits the brisk and resourceful Lucy Eyelesbarrow, an Oxford-educated professional housekeeper, to take a post inside the household and look for what the authorities missed. Lucy becomes Miss Marple's eyes and hands among the Crackenthorpes, observing the ailing patriarch, his watchful daughter, his quarrelling sons and the family doctor while quietly hunting for the proof that a killing took place. As the search narrows and a corpse is finally found, old resentments over money and inheritance rise to the surface, and the question of who could have committed a crime no one was meant to see grows steadily more dangerous.