Miss Marple believes she's seen a murder in a passing-by train, yet when the police find no evidence she decides to investigate it on her own.

Marcelle de Barthas is young French widow, who since her husband’s death some seven years previously, has lived a secluded life in her country house, with her four children. The eldest one, Emmy, is a beautiful and pure young girl of seventeen, who tentatively believes she has a calling to the religious life. The second child, Bertrand, aged fifteen, has been sent to England for an exchange holiday with a young English boy, Harry Fanning, and when the play opens, the French children are excitedly awaiting his arrival. Also living with the family is a French governess, and a tutor, Blaise Lebel. Lebel has a sombre power over the family, and it is the ‘intrusion’ of Harry that sets in motion in him a wave of resentment and fear.