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Having attended General Eisenhower's final briefing on the Normandy landings (D-Day), U.S. Army Major Jeff Pike (James Garner) is sent to Lisbon to confirm with an informant that the Nazis still expect the invasion in the wrong place. However, Pike falls into a trap; he is drugged into unconsciousness and transported to Germany.
When Pike wakes up, he is in what seems to be a U.S. Army Hospital. His hair is graying, and he needs glasses to read. He is told it is six years later and the hospital is in Occupied Germany, even though he has no memory of the intervening period. An Army psychiatrist, Major Walter Gerber (Rod Taylor) explains that he has been having episodes of memory loss for the past few years, ever since he sustained physical trauma in Portugal in June 1944. He advises Pike not to worry, as his blocked memories have always resurfaced within a few weeks, helped along by a treatment that mostly consists of remembering events prior to Lisbon and then pushing on into the blank period. Gerber is assisted by a nurse, the dispassionate Anna Hedler (Eva Marie Saint). To support the illusion that he has been a hospital patient for some time, Pike is provided with letters supposedly written by his father and photos of his parents—Gerber has been researching Pike for many months to prepare for this event—and Hedler tells Pike she is his wife.
Pike is completely taken in and is gratified that his pre-Lisbon memories, at least, are intact and clear. For instance, he remembers the D-Day briefing as if it happened only yesterday, which, of course, it did. As part of the therapy, he recounts the details of the invasion plans, including the all-important location of Normandy (rather than Pas de Calais, as believed by the German High Command) and the date, June 5.
When Pike notices that a nearly invisible paper cut he got in 1944 has not healed yet, he realizes that it is a hoax. Gerber, as it turns out, is a German-American who had returned to the Fatherland to serve the Nazi cause. He likes Pike and readily admits the deception. He says he originally developed genuine techniques to treat amnesia in young soldiers returning from the Russian Front; but they had been perverted to this purpose. Pike's hair had been dyed, of course; and an injection of atropine had impaired his close vision. However, when Pike claims he knew the truth all along and his statements about Normandy were a cover story, Gerber is skeptical.
With the assistance of Anna, who was recruited from a concentration camp because she was a nurse and spoke English, Pike also convinces SS Officer Schack (Werner Peters) that he knew all along it was a ruse. Schack now believes the invasion will be at Calais. Gerber, though, does not, so he plays one last trick, setting the clock in Pike's room ahead several hours. When Pike thinks the invasion has already begun, he lets his guard down and confirms Gerber's suspicions about the Normandy invasion. Gerber then sends an emergency dispatch, which Schack intercepts and disregards, even suggesting Gerber may be a double agent. As it happens, the weather is too rough; and Eisenhower postpones the invasion a day, discrediting Gerber, and Schack orders Gerber's arrest.
Gerber knows that Schack will return to kill them when the Normandy information proves correct, so that his blunder is not revealed. The doctor secretly lets Anna and Pike go, asking Pike to take his psychological research papers on true amnesiacs with him to the West. When he hears the news of the Normandy landing, he takes poison. When Schack shows up, Gerber tries to shoot him but dies too soon. Schack pursues the escaped couple alone, ordering his men to follow when they are assembled.
During their escape, Anna tells Pike of the abuse in the camp, which has left her emotionless. She and Pike go to the local minister, where they are referred to a frankly corrupt, middle-aged German border guard, Sgt. Ernst (John Banner), who is willing to help them cross into Switzerland in return for Pike's watch and Hedler's gold ring. Ernst gives the minister's housekeeper, Elsa (Celia Lovsky), the ring. After the couple and Ernst head for the border, Schack shows up at the manse. When he sees Hedler's gold ring on Elsa’s finger, he forces her to tell him where to find the escapees. Schack catches up with Pike and Hedler at the border, but Ernst shoots him because he doesn't want Schack to mess up his human-smuggling business. Ernst and Pike arrange Schack’s body to make it look as if he had been killed while trying to escape.
Safely in Switzerland, Pike and Hedler are put in separate cars. Pike is told he will be taken to the U.S. Embassy, while Hedler's fate is uncertain. Hedler cries, her first display of emotion in years. In the final scene, the cars come to a fork in the road, with one turning left, to the Embassy, and the other turning right, to a refugee camp.