Seventeen-year-old Richard and his parents take their annual seaside holiday in a guesthouse on England's east coast in the 1950s. Julia, a teenage girl holidaying with her parents in a nearby guesthouse, catches Richard's eye, but her Dutch friend Anna is intent on causing trouble.

The residents of a quiet English village begin to receive nasty, threatening letters. The wife of the local vicar calls in her friend Miss Marple to investigate.

Miss Marple, the spinster detective who is one of the most famous characters created by English crime writer Agatha Christie, is portrayed by Joan Hickson who starred in a dozen television mysteries about Miss Marple over the course of a decade.