I loved this film. It is so genuine, all the persons playing part in it are so very much alive and convincing, that you get under the skin of them all, whether they end up badly or not, even the spies and the crooks. This is not the only war film Winston Churchill wanted to ban, there were a number and several of the best, and this is one of them. Fortunately he didn't succeed in banning any of them.
Especially so long afterwards, 76 years later to be exact, it's immensely rewarding to see such an example of supreme realism all the way, of ordinary people, officers and soldiers, spies and victims, in their very various precarious situations, all under severe pressure, some under threats of death or worse, but all keeping on working and straining themselves for what everyone of them believes is for the best of all. The Germans are not depicted as crooks and villains, they are rather very well objectively filmed, like also the Britishers. They are all doing an extremely difficult job under extreme strain, and this was during the year when the war reached its deepest crisis. It is almost perfectly documentary in character all the way.