Belgian detective Hercule Poirot believes he's on a vacation taking a luxurious cruise down the Nile, but when a newlywed heiress is found murdered on board, he must take up the mantle of detective once again.
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".
Emily Boynton, stepmother to the three Boynton children – Lennox, Raymond, and Carol – and mother to Ginevra, blackmails the family lawyer, Jefferson Cope, into destroying a second will of her late husband that would have freed the children from her dominating influence and allowed them to inherit $200,000 each.