The writers deviated from historical fact (which is fine, it's not a documentary) to make an entertaining story of this mystery. I especially admire it for including one big, pointless plot thread simply because doing so deepens the sense of reality, and adds a bit of comedy. The thread is a sort of red herring, but it has nothing to do with solving the mystery: Honor Blackman is a young woman whose mother is nagging her to press Dirk Bogarde toward marriage. In a more formula-driven film, that would set up tension between Blackman and Jean Simmons for Bogarde's affections. Not here. Blackman and her mother disappear about midway through the movie, never to return, while Bogarde gets down to the serious business of finding Simmons' missing brother. How refreshing is that?!
I generally don't worry about spoilers-- the quality of a good film (especially a mystery) has more to do with how the plot develops than with how it ends-- but in this case, with an solution so unexpected, so ambiguous, and so realistic (compared to the similar "The Lady Vanishes," a marvelous but fanciful film), this really is a movie that must not be spoiled.
I generally don't worry about spoilers-- the quality of a good film (especially a mystery) has more to do with how the plot develops than with how it ends-- but in this case, with an solution so unexpected, so ambiguous, and so realistic (compared to the similar "The Lady Vanishes," a marvelous but fanciful film), this really is a movie that must not be spoiled.