The film presents itself as an intelligently made suspense story, showcasing Jean Simmons’s strong performance as a young woman traveling with her brother "Johnny" (David Tomlinson) to Paris during the Great Exposition. A puzzling situation arises on their first morning: her brother has vanished, and so too has his hotel room! Following in the vein of "The Lady Vanishes" (1938), no one at the hotel—neither employees nor other guests—remembers having seen him, leading to suspicions that she might be either delusional or attempting to evade payment. The situation begins to clarify when Honor Blackman ("Rhoda") tries to return fifty francs her friend "George" (Dirk Bogarde) had borrowed from the missing man the previous evening. With painter “George’s” help, they set out to uncover what's happening. Apart from a disappointing resolution that doesn’t quite fit with the rest of the story, this is an attractive and engaging drama featuring a talented cast, solid writing, and appealing Parisian settings that effectively build suspense. Simmons does a commendable job portraying her character, blending the initial excitement of a young woman in Paris with the fear for her brother and her own mental state. It's definitely worth seeing.

Source on TMDB