A hiker finds a dead woman on the Yorkshire moors; the victim, who has been strangled, is identified as Alice Ruber. Shortly afterwards, Belgian detective Hercule Poirot (Peter Ustinov) examines a diamond belonging to millionaire industrialist Sir Horace Blatt (Colin Blakely), and declares it a fake. Assured that Sir Horace gave a real diamond to his mistress and she later returned the fake to him, Poirot agrees to meet Sir Horace at an exclusive Adriatic Sea island resort to confront her. The hotel is the former summer palace of the reigning King of Tyrania, now owned by Daphne Castle (Maggie Smith), who had received the palace "for services rendered".