The man who knows too much is American doctor Ben McKenna. On holiday in Morocco with his wife Jo and his eight-year-old son Hank, he inadvertently witnesses a murder in a market and becomes privy to a plot of political assassination. This being masterminded by the Draytons, an English couple they have met in a restaurant, and is due to take place in London in a few days' time. In order to prevent McKenna from going to the police, the couple kidnap Hank. Ben is warned not to tell the police what he knows. Hank's life depends on his silence.