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During the Korean War, members of a U.S. Army patrol are captured and taken to Manchuria by Chinese Communists who brainwash them into believing that Raymond Shaw, a mother-dominated sergeant, has led a successful action against the Communists.

Back in the United States, Raymond is awarded the Congressional Medal of Honor on the strength of his comrades' testimony.  Actually he is now a puppet of the Communists; at the sight of a Queen of Diamonds, his mind is triggered into obeying any instruction, retaining no knowledge of his subsequent actions.

Meanwhile, another member of the patrol, Bennett Marco, begins having nightmares in which he vaguely recalls what happened in Korea.  Suspicious, he starts an investigation and discovers Raymond's strange reaction to the playing card.  Unknown to Raymond, the key Communist behind the U.S. operation is his mother, a politically ambitious woman who plans to have her son shoot the presidential nominee during a rally at Madison Square Garden, thus paving the way for her husband, Senator Iselin, the Vice-Presidential nominee, to take control of the government.  As part of the Communist master plan, she uses the cards to force Raymond into killing both his wife, Jocie, and his father-in-law, Senator Jordan, a crusading liberal and his mother's chief political enemy.

On the night of the rally, Marco confronts Raymond with a handful of the cards and tries to convince him that he no longer has control of his own mind, but Raymond follows his mother's instructions and takes a rifle to a deserted projection booth.  At the last moment, however, the hypnotic spell breaks and he kills his mother and stepfather and then takes his own life.