Stephen Neale is released into WWII England after two years in an asylum, but it doesn't seem so sane outside either. On his way back to London to rejoin civilization, he stumbles across a murderous spy ring and doesn't quite know to whom to turn.
I wonder if the shopkeeper who rents Stephen and Carla the room above the bookshop was an inspiration for Mr Charrington in George Orwell's Nineteen Eighty-Four. Anyway, despite Graham Greene's dislike for Alfred Hitchcock, I see him most clearly in the plot hinging on mistaken identity; the idée fixe of Hitchcock who made this theme a rich vein of existential meaning. Indeed, speaking of Hitchcock, Glenn Kenny suggested that:
Nazism […] is arguably reduced to what Alfred Hitchcock called a MacGuffin—that is, an it-could-be-anything pretext for the suspenseful action.
Indeed, it's difficult to see all that much of Lang in this film except in the lighting and the anti-Nazi sentiment, although Joseph Jon Lanthier observed in Slant that:
Nearly the entire carnival accosts him with cold, purposeful eyes, insisting that a mistake has been made. Converting a throng with evidently individual concerns into a mob with a singular, morbid objective was for Lang an activity akin to snapping the fingers.
For me, though, the eerie feeling of 'nothing making sense' begins to wear a little thin after a while and this "minor work in need of some delicate defense" (Lanthier again…) doesn't quite come together.