4:50 from Paddington (Miss Marple)

For an instant the two trains ran together, side by side. In that frozen moment, Elspeth McGillicuddy witnessed a murder. Helplessly, she stared out of her carriage window as a man remorselessly tightened his grip around a woman's throat. The body crumpled. Then the other train drew away.

Elspeth was not a woman usually given to hallucinations, yet it appears that no one else witnessed the murder, no one reported it, and no corpse is found. That's when Elspeth turns to her old friend Jane Marple to help solve the puzzle and unmask a murderer.