Evil Under the Sun (1982)

Hercule Poirot (Sir Peter Ustinov) is called in to investigate a case for an insurance company regarding a dead woman's body found on a moor, and then an important diamond sent to the company to be insured, turns out to be a fake.

Poirot discovers that the diamond was bought for Arlena Marshall (Dame Diana Rigg) by Sir Horace Blatt (Colin Blakely), and Arlena is on her honeymoon with her husband and stepdaughter on a Mediterranean island hotel. He joins them on the island and finds that everybody else starts to hate Arlena for different reasons, refusing to do a stage show, stopping a book, and for having an open affair with Patrick Redfern (Nicholas Clay), another guest, in full view of his shy wife. So it's only a matter of time before Arlena turns up dead, strangled, and Poirot must find out who it is.