John Gielgud brings a certain appeal to his role as a writer who finds himself working for British intelligence, tasked with going to Switzerland to track down a Nazi spy. He’s joined by two helpers: Madeleine Carroll, playing "Elsa," who seems rather innocent but is eager to assist, and Peter Lorre, cast as “The General,” embodying a questionable nature alongside considerable danger. Their initial effort to complete the assignment doesn't quite go according to plan, resulting in them mistakenly targeting an innocent individual. This is when they begin to consider the ethics of their undertaking, especially after uncovering who the actual enemy agent turns out to be. Hitchcock and Charles Bennett adapted Somerset Maugham’s story, paying close attention to both the specifics and the personalities involved; the filmmaking feels tight, and the suspense during the train sequence near the end makes for a captivating experience.

Source on TMDB