Major Bennett Marco, Sergeant Raymond Shaw, and the rest of their infantry platoon are kidnapped during the Korean War in 1952. They are taken to Manchuria, and are brainwashed to believe that Shaw saved their lives in combat– for which Congress awards him the Medal of Honor.
Years after the war, Marco, now back in the United States working as an intelligence officer, begins suffering the recurring nightmare of Shaw murdering two of his comrades, all while clinically observed by Chinese and Soviet intelligence officials. When Marco learns that another soldier from the platoon also has been suffering the same nightmare, he sets to uncovering the mystery and its meaning.
It is revealed that the Communists have been using Shaw as a sleeper agent, a guiltless assassin subconsciously activated by seeing the "Queen of Diamonds" playing card while playing solitaire. Provoked by the appearance of the card, he obeys orders which he then forgets. Shaw’s KGB handler is his domineering mother Eleanor, a ruthless power broker working with the Communists to execute a "palace coup d’état" to quietly overthrow the U.S. government, with her husband, McCarthy-esque Senator Johnny Iselin, as a puppet dictator.
Marco discovers the trigger of the "Queen of Diamonds" and meets with Shaw at the Central Park Zoo shortly before the party's national convention. He uses the card to interrogate Shaw as to his final plan; Shaw is to shoot the presidential candidate during the convention in order to win overwhelming support for Senator Iselin, the vice-presidential candidate, and the dictatorial powers he'll request following the assassination. Marco reprograms Shaw, although the reader is unsure until the final pages if it worked. At the convention, Shaw instead shoots his mother and Senator Iselin. Marco is the first of the authorities to reach Shaw's sniper nest just after Shaw kills himself.