Towards Zero

S3, E3: Lady Tressilian hosts a house party on her Devon estate where loyal companion Mary and her eccentric old friend Treves help their hostess entertain her guests. Miss Marple, an old school friend of Lady Tressilian, is intrigued when Treves tells a story about a child murderer who was never convicted. When Treves is found dead the next day and another murder follows, Miss Marple is determined to find the killer.