Kay Eberstern, English-born, living in Denmark and married to Bror, landowner, farmer, nice guy and proud Dane. They have two grown children, and in their forties are enjoying their lives in the way that you do when you are happy and nothing is rocking the boat. But that is before WW2. When German soldiers begin to appear on the streets of Copenhagen and beyond, Denmark becomes a split country. For many Danes think it is a good thing that Germany is with them; they will be untroubled and protected. Others can see the problems that will and do quickly appear; those that report friends and neighbours to the occupiers are the danger for those loyal Danes who don't want Hitler's men in their country. Which side are we on, then? Kay Eberstern does not want to take sides at all until her husband has signed, early in the occupation, a document that confirms he is happy to deal with the Germans. The rift this causes is a terrible thing for a once happy marriage, and certainly when Kay offers to help a secret agent hide for a few days, we know that things are never going to be the same.
There is something else important about this novel, for woven between the Danish chapters there is the story of those who didn't join any army, but served their country silently - the women who would never have been code breakers and cypher clerks in Britain if the men were not already in uniform and fighting the enemy. And the agents Britain trained for undercover work in Denmark because the allies were not sending forces there.
I found this a very readable book; at around 500 pages I finished it in two days, not stopping much in a marathon read. Buchan's style is easy despite a lot of technical detail in the chapters about code and cypher staff. Don't be put off, it was fascinating. I always enjoy a novel that has fact woven in. Good research builds a good story. One of those "phew! what a read" kind of books!