A young doctor, who has returned from an Arctic expedition, learns that he is the only witness that could have saved a young prisoner accused of the murder of his adoptive mother from the gallows. He decides to inform the family of the deceased with the news, but realizes that it has re-opened old wounds and that they are upset with him for bungling into their now-happy life. As the finger of suspicion points from one person to the next, he decides to prove the young man's innocence. Miss Marple, who is staying with the family for the wedding of the father and his secretary, also assists in the investigations. But the true killer becomes restless, striking once more and cutting the threads of happiness in the family. Miss Marple reveals the truth, but not before the ordeal by innocence claims another life.