13 Rue Madeleine (movie, 1947)

Bob Sharkey, an instructor of would-be spies for the Allied Office of Strategic Services, becomes suspicious of one of the latest batch of students, Bill O'Connell, who is too good at espionage. His boss, Charles Gibson confirms that O'Connell is really a top German agent, but tells Sharkey to pass him, as they intend to feed the mole false information about the impending D-Day invasion.