The Man Who Knew Too Much (1956)

Dr. Ben McKenna, his wife Jo and their son Hank are on a touring holiday of Africa when they meet the mysterious Louis Bernard on a bus. The next day Bernard is murdered in the local marketplace, but before he dies he manages to reveal details of an assassination about to take place in London. Fearing that their plot will be revealed, the assassins kidnap Hank in order to keep the McKenna's silent. Ben and Jo go to London and take matters into their own hands.