While on an extended European vacation following a medical conference in Paris, American physician Ben McKenna and his family are traveling on a bus in French Morocco when his young son Hank accidentally pulls the veil off a Moslem woman. An international incident is avoided when Louis Bernard, a Frenchman, intercedes on the McKennas' behalf. While Ben is happy to tell Louis all about his family and their planned excursions in Marrakech, his wife Jo is suspicious of the Frenchman's constant questioning. That night, Louis meets the McKennas in their hotel room for dinner, but suddenly cancels their supper plans when Rien, a hired assassin, appears at the McKennas' door. Later, at an Arab restaurant, Ben and Jo meet a British couple, the Draytons, who claim to be fans of Jo, who was a well-known singer prior to her marriage to Ben. The next morning, the McKennas and Draytons meet at the local marketplace. The usual frivolity of the market crowd is broken when a man being chased by the police collapses in Ben's arms, having been fatally stabbed in the back. It is Louis, disguised as an Arab, who, with his dying breaths, tells the physician that there is a plot to assassinate an unnamed statesman in London. While the McKennas are taken to the police station for questioning, Mrs. Drayton agrees to care for Hank in their absence. In the midst of being interrogated, Ben receives a phone call from a kidnapper, threatening his son with grievous harm if he tells the authorities what Louis said to him. After giving the high-strung Jo a sedative, Ben informs his wife that their only son has been abducted. Aware that the Draytons left Marrakech on a private plane, Ben and Jo decide to go to London and search for Hank there. Greeted at the airport by both Jo's fans and the police, the McKennas are interviewed by Inspector Buchanan of Scotland Yard, who tells them that he is well aware of the reason why their son was kidnapped. Despite his wife's pleas, Ben refuses to tell the inspector what Louis said to him, claiming that the British secret agent had spoken to him in French. Jo then receives a phone call from Mrs. Drayton, who allows the McKennas to briefly speak to Hank. Checking into a London hotel, the McKennas attempt to call Ambrose Chappell, the name Louis told Ben, but they are interrupted by the arrival of Jo's old acquaintances: Val and Helen Parnell, Jan Peterson and Cindy Fontaine. While Jo stays behind with her friends, Ben sneaks out through the hotel's service entrance to meet Chappell. At the Ambrose taxidermy shop, Ben is slow to realize that neither Ambrose Sr. nor Ambrose Jr. is involved in his son's kidnapping, and is forced to make a quick escape before the police arrive. Meanwhile, at the hotel, Jo realizes that "Ambrose Chapel" is a place, not a person, and she is soon met there by Ben. Inside the church, Hank is being held captive by the Draytons, with the help of their assistant, Edna. Also there is Rien, who is being instructed by the Draytons as to the exact moment during an Albert Hall concert that he is to commit the assassination: at a climactic cymbal crash in the performance of a cantata. The McKennas enter the chapel just as the service, administered by Mr. Drayton, is about to begin. While Ben stays inside, Jo leaves to call the police, so the Draytons cut short the service. Hearing his son's voice, Ben rushes to Hank's aid, only to be knocked unconscious by one of Draytons' henchmen. By the time the police arrive at the chapel, the Draytons have made their escape with Hank. Refusing to enter the locked church without a search warrant, the police leave, so Jo calls the police station, begging for help. She asks to speak to Buchanan, but is told that he is at an important diplomatic function at a concert at Albert Hall. When the policeman refuses to contact Buchanan, she heads off alone to Albert Hall to find him. When Rien sees her there, the assassin reminds Jo that Hank's safety depends on her silence. Meanwhile, Ben escapes the locked chapel by climbing the church bell's rope and also makes his way to the concert. Realizing that Rien is about to shoot a visiting foreign prime minister, Jo screams, causing the startled assassin to merely wound the dignitary in the arm. Ben then jumps Rien, and in his attempt to escape, the assassin falls from the balcony to his death. Back at the embassy, the Draytons are informed by the ambassador that their assassination attempt on the prime minister has failed. Despite Mrs. Drayton's objection, the ambassador then orders her husband to kill Hank. With the police unable to go into the embassy due to diplomatic immunity, the McKennas enter alone, as the invited guest of the grateful prime minister. Jo is asked to perform for the guests, and her singing voice is soon recognized by Hank. Under Mrs. Drayton's instruction, the young boy whistles along with her singing, guiding Ben to the room in which his son is being held. Mr. Drayton then appears, but rather than killing them, he decides to use Ben and Hank as human shields during his escape from the embassy. As they make their way down the grand staircase, Ben pushes Drayton, and the spy is killed when he falls on his own gun. The reunited McKennas then head back to their hotel room, where Jo's friends have been waiting the entire time.