Warning: Wikipedia contains spoilers
The premise of the film was that the Chinese Communists in the 1950s had developed a technique (akin to hypnosis) whereby a person could be snapped into and out of a trance, ordered to do things with full compliance, and have no memory of such actions afterwards. To cover their tracks, the Communists would implant false memories in the American soldiers' heads and provide a subconscious trigger whereby the soldier could be snapped into and out of hyponosis. Even after full reintegration with American society, they would have no knowledge of their having been brainwashed or the triggers which set them off.
The movie stars Frank Sinatra (as Major Bennett Marco) and Laurence Harvey (as Sergeant Raymond Shaw) army officers who suffered so-called "brainwashing" during the Korean War. Their squadron is made to believe Raymond Shaw saved their lives, for which he receives the Medal of Honor when they return to America. In actuality, the Communists intended to use Raymond as a test sleeper agent abroad and, using the Queen of Diamonds as a subconscious trigger, compel him to do heinous crimes, including murder. It is learned late in the movie that Raymond was, in fact, controlled by his mother (played by Angela Lansbury) who saught to advance the fortunes of her husband and Raymond's step-father, Joseph Iselin (played by James Gregory), a McCarthy-like senatorial demagogue who was running for President and believed to have been a caricature of Richard Nixon.
Frank Sinatra plays Bennett Marco, a war-time friend of Raymond's, who has a recurring dream in which Raymond kills two of his fellow Army mates. When he finds out that another Army member has been having the same dream, he sets out to uncover the mystery. Janet Leigh plays Bennett's love interest.
The Manchurian Candidate is a psychological drama, a thriller, and a black comedy combined, much as was Dr. Strangelove.
It won acclaim for its politican themes and the exploration of the connection between the far left and far right in cold war America.
The film is consistently in the top 100 on the Internet Movie Database's list of top 250 films; was #67 on the American Film Institute's 100 Years, 100 Movies, and #17 on its 100 Years, 100 Thrills; and has been deemed "culturally significant" by the Library of Congress and selected for preservation in the United States National Film Registry.