Superintendent Macdonald, CID, studied his fellow-passengers on the Vienna plane simply because he couldn't help it, because he hadn't conditioned himself to being on holiday. The distinguished industrialist he recognised: the stout man he put down (quite mistakenly) as a traveller in whisky. The fair girl was going to a job (he was right there) and the aggressive young man in the camel coat might be something bookish. Macdonald turned away from his fellow-passengers deliberately; they weren't his business, he was on holiday - or so he thought.