The film won an Academy Award for Best Song for "Que Sera, Sera (Whatever Will Be, Will Be)", sung by Doris Day. It premiered at the 1956 Cannes Film Festival, on April 29.
An American family – Dr. Benjamin "Ben" McKenna (James Stewart), his wife, popular singer Josephine Conway "Jo" McKenna (Doris Day), and their son Henry "Hank" McKenna (Christopher Olsen) – are vacationing in Morocco. Traveling from Casablanca to Marrakesh, they meet Frenchman Louis Bernard (Daniel Gélin). He seems friendly, but Jo is suspicious of his many questions and evasive answers.
Bernard offers to take the McKennas out to dinner, but cancels when a sinister-looking man knocks at the McKennas' hotel-room door. Later, at a local restaurant, the McKennas meet friendly English couple Lucy (Brenda De Banzie) and Edward Drayton (Bernard Miles). The McKennas are surprised to see Bernard arrive and sit at another table, apparently ignoring them.
The next day, attending a busy Moroccan outdoor market with the Draytons, the McKennas see a man being chased by police. After being stabbed in the back, the man approaches Ben, who discovers it is actually Bernard in disguise. The dying Bernard whispers that a foreign statesman will be assassinated in London soon, and that Ben must tell the authorities there about a name that Ben writes down as "Ambrose Chappelle". Lucy offers to return Hank to the hotel while the police question Ben and Jo, accompanied by Edward. An officer explains that Bernard was a French Intelligence agent on assignment in Morocco.
In a phone call, Ben is told that Hank has been kidnapped but will not be harmed if the McKennas say nothing to the police about Bernard's warning. Knowing that Hank was left in the care of Lucy, Ben dispatches Edward to locate them. When Ben and Jo return to the hotel, they discover Edward has checked out.
It is then that Ben figures out that the Draytons are the couple that Louis Bernard was looking for and they are involved in Hank's abduction. When he learns that the Draytons are from London, he decides that he and Jo should go to London and try to find them – and Hank – through Ambrose Chappelle.
In London, Scotland Yard's Inspector Buchanan (Ralph Truman) tells Jo and Ben that Louis Bernard was in Morocco trying to uncover an assassination plot, and that they should contact him if they hear from the kidnappers. Leaving friends in their hotel suite, the McKennas search for a man named "Ambrose Chappelle", but that turns out to be a wild goose chase. Jo realizes that they should be looking not for a person but a place: "Ambrose Chapel", and that is where the McKennas find Edward Drayton leading a service. Jo leaves the chapel to call the police. After Drayton sends his parishioners home, Ben confronts Drayton, and is knocked out and locked in the chapel. Jo arrives with police at the now-locked chapel, but they cannot enter the chapel without a warrant.
Jo learns that Buchanan has gone to a symphony concert at the Royal Albert Hall, and she asks the police to help her get to the Hall. Once the police and Jo leave, the Draytons sneak Hank out of the rear of the chapel and take him to a foreign embassy. Meanwhile, in the Albert Hall's lobby, Jo sees the sinister man who came to her door in Morocco. When he threatens to harm Hank if she interferes, she realizes that he is the assassin sent to kill the foreign Prime Minister (Alexis Bobrinskoy) at the concert.
Ben, having escaped the chapel through the bell tower, follows Jo to the hall, where she points out the assassin. Ben searches the balcony boxes for the killer, who is waiting for a cymbal crash to mask his gunshot. As the cymbal crashes, Jo screams and the assassin misses his mark, only slightly wounding his target. Ben struggles with the would-be killer, who falls to his death from the balcony.
Concluding that Hank is likely at the embassy, but that the embassy is sovereign and, thus, exempt from an investigation, the McKennas devise a scheme to secure an invitation from the grateful Prime Minister. The ambassador (Mogens Wieth) himself organized the plot to kill the Prime Minister, and blames the failed attempt on the Draytons. Also knowing the McKennas foiled his plan, he orders the Draytons to hide Hank and make preparations to kill him.
The Prime Minister asks Jo to sing. She loudly performs "Que Sera, Sera (Whatever Will Be, Will Be)", so that Hank will hear her. Lucy is guarding Hank, but having second thoughts about the plan, tells him to whistle along with the song. Ben finds Hank. Drayton tries to escape with them at gunpoint, but when Ben hits him, he falls and dies accidentally.
The McKennas return to their hotel suite. Ben explains to their now-sleeping friends, "I'm sorry we were gone so long, but we had to go over and pick up Hank."
Alfred Hitchcock's cameo is a signature occurrence in most of his films. In The Man Who Knew Too Much he can be seen 25:42 into the film, in the lower left corner, watching acrobats in the Moroccan marketplace, with his back to the camera, wearing a light gray suit, and putting his hands into his pockets, just before the spy is killed.
Alfred Hitchcock first considered an American remake of The Man Who Knew Too Much in 1941, but only brought back the idea in 1956, to make a film that would fulfill a contractual demand from Paramount Pictures. The studio agreed it was a picture that could be well-adapted to the new decade. Screenwriter John Michael Hayes was hired on the condition that he would not watch the early version or read its script, with all the plot details coming from a briefing with Hitchcock. Only the opening scenes of the script were ready when filming begun, and Hayes had to send by airmail the subsequent script pages as he finished them.
Hitchcock again brought James Stewart to be his protagonist as he was considering the actor a creative partner, and Paramount wanted a sense of continuity between his works. The director requested blonde Doris Day for the main female role as he liked her performance in Storm Warning, though associate producer Herbert Coleman was reluctant on Day, whom he only knew as a singer. Coleman strongly suggested that the more serious blonde actresses like Lana Turner, Grace Kelly, or Kim Novak be cast in the role, or a suitable brunette, like Jane Russell, Gene Tierney, or Ava Gardner. However, Day was eventually cast in the female lead.
The Albert Hall sequence drew some inspiration from H. M. Bateman's comic "The One-Note Man", which followed the daily life of a musician who only plays one note in a symphony, similar to the cymbal player in the film.
Hitchcock's frequent composer, Bernard Herrmann, wrote the "background" film score; however, the performance of Arthur Benjamin's Storm Clouds Cantata, conducted by Herrmann, is used as source music for the climax of the film. In addition, Doris Day's character is a well-known, now retired, professional singer. At two points in the film, she sings the Livingston and Evans song "Que Sera, Sera (Whatever Will Be, Will Be)" which won the 1956 Best Song Oscar under the alternate title "Whatever Will Be, Will Be (Que Sera, Sera)". The song reached number two on the US pop charts and number one in the UK.
Herrmann was given the option of composing a new cantata to be performed during the film's climax. However, he found Arthur Benjamin's cantata Storm Clouds from the original 1934 film to be so well suited to the film that he declined, although he did expand the orchestration, and insert several repeats to make the sequence longer. Herrmann can be seen conducting the London Symphony Orchestra with mezzo-soprano Barbara Howitt and chorus during the Royal Albert Hall scenes. The sequence in Albert Hall runs for twelve minutes without any dialogue, from the beginning of Storm Clouds Cantata until the climax, when Doris Day's character screams.
The film was a commercial success. Filmed on a budget of $1.2 million, it grossed $11,333,333 at the domestic box office, earning $4.1 million in US theatrical rentals.
In 2004, American Film Institute included the song "Que Sera, Sera (Whatever Will Be, Will Be)" as #48 in AFI's 100 Years...100 Songs.
The Man Who Knew Too Much was kept out of re-release until 1983 when it was purchased by Universal Pictures. The film has been released on home video by Universal Pictures in VHS, DVD and Blu-ray formats. The 2000 DVD includes a special documentary on the making of the film, including interviews with Hitchcock's daughter, Patricia Hitchcock, and members of the production crew.