Prior to the United States entering WWII, the FBI is monitoring known and suspected Nazi agents in the country. Purely by happenstance, they discover a Nazi cell in the US trying to smuggle information back to Germany on Process 97, a key part of the American atomic bomb development. The FBI's Inspector George Briggs (Lloyd Nolan) leads this case, known as the Christopher Case as the unknown leader of the Nazi cell has the code name Mr. Christopher. Briggs' team cannot just bring in the known Nazi agents, but rather they have to find out exactly how the information is being leaked and who Mr. Christopher is. They co-opt the assistance of William Dietrich (William Eythe), a German-American university student who the Nazis tried to recruit. As such, he will be working for the FBI as a double agent. Unknown on either side, it becomes a race as Dietrich's Nazi contacts in the United States, led by Elsa Gebhardt (Signe Hasso), don't fully trust him and come ever so close to discovering his double agent status, while Briggs, Dietrich and their associates close in on the leak and the identity of Mr. Christopher. The modi operandi for both sides change slightly with December 7, 1941.