The House on 92nd Street (1945)

Preface: a stentorian narrator tells us that the USA was flooded with Nazi spies in 1939-41. One such tries to recruit college grad Bill Dietrich, who becomes a double agent for the FBI. While Bill trains in Hamburg, a street-accident victim proves to have been spying on atom-bomb secrets; conveniently, Dietrich is assigned to the New York spy ring stealing these secrets. Can he track down the mysterious "Christopher" before his ruthless associates unmask and kill him?