Crime and revenge - Agatha Christie

Miss Marple read in complete astonishment the letter sent to her by a recently deceased Mr. Raphael, an acquaintance of hers, whom she had met a few times on her previous travels. In this letter, he left her instructions to investigate a murder that had taken place before his death. The only problem I faced was that he did not disclose the identity of those involved in this crime, nor the place or time in which it was committed.