Distant cousins and childhood friends Elinor Carlisle and Roddy Welman are happily engaged to be married. One day Elinor receives an anonymous letter in the post, claiming that someone is trying their hardest to work their way into the affection, and subsequently also bank account, of her wealthy aunt Laura Welman, who is presently bedridden at her home since after a stroke and from whose death Elinor and Roddy both expect to inherit a large fortune. Not knowing what to make of the letter, the young couple eventually decide to pay Laura a visit in order to see for themselves what is really going on at the house. When they arrive, Elinor immediately becomes suspicious towards Mary Gerrard, the lodge keeper's daughter, recently returned to England after having studied in Europe and whom everyone else at the house seem to absolutely dote on. No one else however seem to share Elinor's suspicions or dislike of the young woman, and in particular not Roddy, whom Elinor one night discovers kissing Mary Gerrard in her aunt's drawing room. When Mary suddenly turns up dead, poisoned by a salmon sandwich, Elinor's dislike of Mary makes her the obvious suspect and hardly has time to take in what has happened before she finds herself on trial and condemned for murder. The only person who is not entirely convinced of Elinor's guilt is Doctor Peter Lord, who decides to contact his old friend Hercule Poirot in order to help him prove her innocence.
