The FBI uses a double agent to uncover German spies getting information on the atom bomb.
A narrator declares that the Federal Bureau of Investigation (FBI) increased its 2,000 employees in 1939 to 15,000 during the war. The Germany embassy is filmed. Bill Dietrich (William Eythe) is recruited to spy for Germany but works for the FBI. Agent George Briggs (Lloyd Nolan) investigates the Christopher case. Germans give Dietrich $50,000 and tell him only Christopher can countermand his orders. His papers are put on microfilm in his watch but are switched by the FBI.
Dietrich calls on Elsa Gebhardt (Signe Hasso) and gives her the microfilm instructions that have been altered to give him access to all agents. Dietrich is questioned and says he will set up a radio station. Elsa says she will check with Hamburg. Briggs sends two men to check on Elsa's house. Dietrich opens an office as an engineer, and Col. Hammersohn (Leo G. Carroll) visits him. A mirror window is used to film them. Hammersohn says the US is testing a new gun and mentions secrets of the P-38 plane. Dietrich asks to meet Adolph Klein and Christopher. Outside New York, Dietrich sets up a transmitter that sends to an FBI station nearby that sends messages to Germany and receives them. Dietrich's visitors are filmed.
After December 7, 1941 all suspected spies are arrested with a few exceptions. Hammersohn takes Dietrich to Klein. He gives Klein microfilm and receives information. Drunk Gus tells about ships and demands money. Gus leaves and is taken away. Dietrich gives Klein money and asks about Christopher. Elsa summons Dietrich and loans him papers from Christopher for one day and says they will also be mailed. Hammersohn tells Elsa that Klein was arrested. She sends Max to cover Dietrich. Briggs calls in a physicist and asks him to change the data to throw them off. The FBI traces lipstick on a cigarette butt to a woman who is questioned by Briggs. He learns that Charles Roper (Gene Lockhart) is getting information out of a lab. Dietrich gets himself stopped by police and taken in so that Briggs can visit him. Dietrich says that Hamburg has memory artists. Briggs questions Roper at the lab and persuades him to cooperate. His latest message was put in a book. The bookstore is watched.
Elsa and Hammersohn learn that Dietrich is forbidden to contact agents. Two spies have taken the message to Dietrich, but they suspect he is sending only a short distance. A third arrives, and they take Dietrich to Elsa, who gives him a truth drug. Briggs says he knows who Christopher is, and the FBI raids the house. An FBI agent calls Elsa and gives them two minutes to surrender. Elsa burns papers, changes clothes, and goes out the window as the FBI explodes gas and breaks in. The narrator says that 16,440 people were arrested, hundreds were imprisoned, and six were executed. Yet no sabotage occurred in the United States, and no war secrets were lost.
The statistics given at the end give some indication
of how many innocent people must have been detained in these extraordinary
measures taken against suspected spies, and claims made about
the FBI's exceptional efficiency are also hard to believe. This
film pioneered a new documentary style of filmmaking and served
as propaganda to justify excessive law enforcement measures used
in war-time.