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 ...
A priceless painting by "van Goff" is stolen from a New York museum and a watchman is killed. The insurance company hires Markham to find it, and a mysterious letter lures him to a town called Santa Maria. Will he find the painting there?
A young woman contacts Markham after moving in with an elderly aunt she hadn't seen since childhood. The niece fears that the old lady is being robbed by an unscrupulous medium whose séances bring back the spirit of the woman's long-dead son. Markham attends one of the sittings and immediately pegs the spiritualist as a phony; he also finds the aunt to be a little too theatrical to be a sheltered old hermit.
A law school friend asks Roy Markham to quash rumors that his lady friend, the Bel-Air resident, killed her husband. The coroner ruled her husband's death a suicide. Roy is able to warn his friend that it was murder.
Markham comes home to find an unconscious, rich heiress and her three armed kidnappers. The crooks demand $500,000 for her return and know that her father will contact Markham for help since he'd worked for the family before. Knowing that they'll be killed after the ransom is paid, Roy schemes to overtake their captors with a well-timed explosion.