Dr. Ben and Josephine McKenna (Jimmy Stewart and Doris Day) are an American couple on an overseas holiday in Morocco with their son, Hank (Christopher Olsen). While in a shopping bazaar in Marrakesh, they witness the assassination of a spy, Louis Bernard (Daniel Gelin), whom they met on a bus the day before. Before dying, Louis passes on to the McKennas some information about an assassination that is to take place in London in just a few days' time. Hank is abducted by assassination plotters to prevent Dr. McKenna from telling the authorities of Louis' secret. After following a series of leads, the couple discover that the assassination is on the orders of an ambassador, and will take place during a concert at the Royal Albert Hall. Ben and Josephine then travel to London to rescue their son and to deal with the assassination plotters themselves.