Tender to Danger — Eric Ambler

A blanket of fog grounds a flight in Brussels and Dr. Andrew Maclaren finds himself sharing a room with a fellow passenger who by morning disappears in suspicious circumstances, leaving behind an envelope hidden under the carpet. Whilst the contents are seemingly innocent, they lead the young Doctor into nightmarish adventures, culminating in the avoidance of cold-blooded killers in a deserted windmill and in the company of a beautiful redhead.