Murder at the Gallop (1963) on Netflix

Miss Marple and Mr. Stringer are collecting money for The Reformed Criminals Assistance League, and the old lady decides to pay a visit to the stingy Old Enderby. They go to his manor and witness him dying frightened to death by a cat. Miss Marple snoops around the house and finds a piece of mud that she keeps with her. She calls the police and later she reports to Inspector Craddock that Old Enderly was murdered. However he tells that the Death Certificate states the cause of his death as heart failure and Old Enderby had had a severe heart condition for many years. Miss Marple decides to investigate one of the heirs but she finds her dead in her house with her housemaid Miss Milchrest. Then she lodges in The Gallop Hotel to investigate who might be the murderer. When her prime suspect is killed, she finds the reason why the assassin is killing the relatives of Old Enderby. Now Miss Marple plots a scheme to catch the killer.