Married couple Bob and Jill Lawrence (Banks and Best) are vacationing in Switzerland with daughter Betty (Pilbeam), when Louis Bernard (Fresnay), a Frenchman who befriends them, is found murdered. Before dying, however, Louis Bernard whispers a secret--that a diplomat will be assassinated
at great embarrassment to the British government. To keep Bob's lips sealed, Betty is kidnapped--to be held until after the assassination by hired killer Abbott (Lorre), scheduled to take place during a concert at London's Albert Hall. Bob must follow his duty as a Englishman and prevent the
assassination, but at the same time he must do all in his power to insure the safety of hi...