Upon hearing that Roy Markham's (Ray Milland's) good friend, Jim Donovan, has been shot and killed on the steps of U.S. Embassy in Cairo, Markham and his partner John Riggs (Simon Scott) fly to Washington to investigate. Markham believes Boris Vascou (Anthony Eustrel), the "Vanishing Assassin," murdered his friend over information Donovan planned to share with the U.S. government about the country where he had been working as an oil engineer: "Kaleda," a small, oil-rich kingdom on the Persian Gulf. Its ruler, Prince Hassan III, is due to arrive in Washington shortly on a diplomatic visit. After crashing the right cocktail party, Roy meets the Prince's emissary, Ramik Sarid (Arnold Moss), and deduces that an assassination is in the works. He guesses it will take place at a riverside home where a beautiful French actress named Renee Compante (Jacqueline Beer) awaits the Prince. With the help of a friendly socialite boater, Diana Rogers (Betsy Jones-Moreland), Markham, Riggs and Sarid waylay Vascou and kill him.
—Malia Kapalena