Foreign Intrigue (1956)

Millionaire Victor Danemore, living on the French Riviera, dies suddenly of a heart attack. His secretary, Dave Bishop, wants to know more about his employer's life. Surprisingly, not even his young wife knows anything about her husband's background or how he earned his fortune. Clues lead Bishop to Vienna and Stockholm, where he learns that Danemore was black-mailing people who cooperated with the Nazis during WW2.