I would really recommend 'Two Headed Spy', a film about Major Scotland, a British spy planted in the German army after WWI, who rose to become Lieutenant General of Logistics under Hitler. Despite what's in the movie, he had a radio, not a contact, according to the book, and made sure the Germans:
- didn't have invasion barges after Dunkirk;
- didn't have winter furs for the Russia campaign;
- didn't have enough fuel for the Battle of the Bulge, and;
- told the allies where the German supply dumps were after the Normandy landings, so they bombed them. Without supplies the Germans couldn't hold the line.
That well placed spy achieved so much.
It used to be available here:
https://archive.org/details/moviesandfilms?noscript=true
But archive.org is down. If you can find it I recommend the book.