The couple, Dr. Benjamin McKenna (played by James Stewart) and his wife Jo (played by Doris Day), are on vacation in Morocco with their son Hank (played by Christopher Olsen). While on a bus tour, they meet Frenchman Louis Bernard (played by Daniel Gélin), who is later killed in front of them at a market. But before he dies, he tells Ben about an assassination plot that is set to take place in London. Unfortunately, Hank is kidnapped by the conspirators to keep the McKennas from going to the authorities.
The McKennas travel to London and are forced to navigate the dangerous world of spies and assassins on their own. They meet a British agent named Buchanan (played by Bernard Miles) and work together to track down the sinister mastermind behind the plot. As they get closer to finding Hank and stopping the assassination, Jo becomes more and more agitated and desperate to save her son, culminating in her famous rendition of the song "Que Sera, Sera."
The Man Who Knew Too Much is a thrilling and suspenseful drama that showcases Hitchcock's signature tension-building techniques. It also features standout performances from Stewart and Day, who display great chemistry and emotional depth in their roles as parents fighting to save their child.