The movie lacks credence with the helicopters which didn't exist until the 1950s. But no woman would do what was done here, even a woman before the women's movement of the 60s and 70s. About the only portion of the movie that you could believe in was that Germany would want to know where the landing would be. Ignore for the moment that the British had captured all the spies but even if they had not, they wouldn't have let one roam around like this just to reassure the Germans that the landing would be at Calais. It isn't one major thing that makes the movie not work. It is the culmination of all the things wrong that makes the movie fail. Bad directing, bad scripts, no attempt at authenticity (at all) all combine to just make the movie fall flat. Generally speaking spies should fade into the woodwork. The suspense comes in with the spy wondering if the information they have is valid or not and worrying about being detected. On this one that game was over from the start. This spy was doing anything but spying. Your only chance at getting something that has some credibility and instills some suspense may be to read the book.