The Lady Vanishes, released in 1938, was Hitchcock’s last British film (until the 1970s), and is considered to be his finest. Hitchcock’s 3 previous films had not done well at the box office, but The Lady was extremely successful in both the U.K. and the U.S.A. and helped launch Hitchcock in Hollywood.
The plot of The Lady Vanishes has clear references to the political situation leading up to World War II. It is hard to imagine yourself back in 1938, before the world would experience the atrocities of the 2nd World War. Spies on trains and coded messages drive the plot.
I recently watched the film and was surprised by how slowly the plot moves in the first third of the film. Even though this film is always rated in the top 40 best British films, for me it was surprising that Hitchcock directed it. But, that is just my opinion and I am no expert! For a more positive assessment of the movie, see this source: https://www.theguardian.com/film/filmblog/2012/jul/24/my-favourite-hitchcock-lady-vanishes