(Here's my reviews of other Hitchcock films: "Rear Window" and "Shadow of a Doubt".)
Hitchcock actually made 2 movies called "The Man Who Knew Too Much" but I've only seen the second one. (Incidentally, Hitchcock had long wanted to remake this movie but what got the project going was the desire to help out an alcoholic friend who was down on his luck.) Jimmy Stewart plays Dr. Ben McKenna who is vacationing with his wife (Doris Day playing a retired singer) and son in Morocco. While traveling on a bus their young son accidentally pulls the veil off a Muslim woman when the bus lurches. A Frenchman named Louis Bernard defuses the situation and they agree to dinner with him that night. But Mr. Bernard acts very strangely and cancels at the last moment, and the next day while the McKennas are shopping in the market he staggers up to them disguised as an Arab and with a knife in his back. Before he dies he whispers a message to Dr. McKenna.
I don't want to give away too much of the story but I love the way Hitchcock pulls the viewer in to a suspenseful story. The concert scene in Royal Albert Hall in London is a perfect example: as the music plays you know the story is leading up to a gunshot to be fired when the cymbals crash during the "Storm Cloud Cantata" (which was lengthened by about a minute and a half for the movie). In fact, music is a central part of the film and it produced Doris Day's biggest hit with "Que Sera, Sera (Whatever Will Be, Will Be)." According to Patrick McGilligan, she didn't even like the song because she thought it was a children's song, and for years she refused to perform it. But it's not all suspense, and Hitchcock's humor shows through several times - most notably at the very end. But at any rate, I highly recommend tracking it down and watching it if you haven't already. Or even if you have!