Justice's Revenge - Agatha Christie

Miss Marple read the letter again. It was a strange letter from Mr. Raphael, who died some days ago, and whom Miss Marple had known many years before when he had helped her solve a crime in the Caribbean.

The letter says that Miss Marple has an unmistakable sense of evil and an unwavering determination to seek justice, and asks her to investigate a murder.

But the letter does not mention anything about the crime or the murderer, so what can she do?