Agatha Christie's
Evil Under the Sun

(1982)

Retired actress and diva Arlena (Diana Rigg) is murdered while she and a small group of friends and collegues are vacationing on an exclusive island in the South of France.  Everyone has a motive to kill her and so Hercule Poirot (Peter Ustinov) launches a quiet investigation to flush out the guilty party.

After interviewing all of the island vacationers and testing their alibis, Poirot gathers everyone together and determines Christine and Patrick Redfern are the murderers.  Patrick was having an affair with Arlena and while Christine acted as though she was devastated by her husband’s filandering ways, in fact they plotted together to kill Arlena, steal the expensive diamond that Arlena kept from her previous relationship, and cash in on it. They were also responsible for murdering a woman at the beginning of the film and using her checking account to pay for their extravagant lifestyle. They are both arrested and ferried off the island by the authorites.

Thanks Sarriesleepers!