The Counterfeit Traitor (1962)
A former American citizen, now living in Sweden, is blackmailed into spying for the Allies during World War II. Reluctant at first, his relationship with a beautiful German agent causes him to realize how important his work is. When he learns that his German friends have been/are being found out, he puts his own life in danger to go back inside Nazi Germany to finish his work and try to save his friends. It's an exciting story with great characters, filmed partly in the locations where the story took place.