Leo Marks, who wrote Peeping Tom, also wrote this story. Anything with Powell and Marks is worth checking out, it's as if they wanted to recreate the old Powell-Pressburger magic.

Marks had been a spook himself, a cryptographer, so he must have had some knowledge of "the game". This one is as much melodrama as thriller, as two enemy spies fall in love with each other; you can imagine Garbo playing this sort of thing in the 1930s.