Murder Most Foul (1964)

Although the evidence appears to be overwhelming in the strangulation murder of a blackmailer, Miss Jane Marple's (Dame Margaret Rutherford's) sole "not guilty" vote hangs the jury 11-1. She becomes convinced that the real murderer is a member of a local theatrical troupe, so she joins them in order to gather information. The clues lead back many years to a single disastrously unsuccessful 1951 performance of a dreadful play