Renowned Belgian detective Hercule Poirot (Peter Ustinov) is called in by a London insurance company when wealthy industrialist Sir Horace Blatt (Colin Blakely) attempts to have a cheap piece of costume jewellery insured for £50,000. Puzzled by this very wealthy man attempting to perpetrate such an obvious fraud, Poirot is sent to meet him on his yacht in the south of France. When the two men meet, Sir Horace tells Poirot that the jewel was a gift to a mistress, former actress Arlena Marshall (Diana Rigg), who has obviously had the original jewellery replaced with a fake. Poirot heads to an exclusive hotel on a small island in the Adriatic to meet Arlena and to investigate. While there he encounters an assortment of wealthy guests, including Arlena and her husband Kenneth (Denis Quilley), her lover Patrick Redfern (Nicholas Clay) and her former associate Daphne Castle (Maggie Smith), who now runs the hotel. Also present are waspish gossip columnist Rex Brewster (Roddy McDowall), Re