Miss Marple: They Do It with Mirrors (1991)

Ruth van Rydock is worried about her sister's health and well-being and asks their mutual friend Jane Marple to visit her for a awhile. Her sister, Carrie-Louise Serrocold, is married to Lewis Serrocold and he has turned their estate into an institute of sorts for young offenders. Miss Marple is more than happy to visit and finds an odd assortment of characters on the premises.

When Christian Gulbranson is found shot while sitting at the typewriter in his room, Miss Marple has a murder to solve. Gulbranson, a trustee of the institute, had only arrived that day on urgent business and Lewis Serrocold found a partly completed letter in his typewriter saying that someone was poisoning Carrie-Louise. Assisting Inspector Slack, with whom she had previously worked, she soon knows who committed the crime but is not quite sure how they did it.