An FBI agent infiltrates a Nazi spy ring.

In 1939, due to increasing hostilities in Europe, the Federal Bureau of Investigation intensifies its observation of foreign nationals living in the United States. The F.B.I. finds a valuable ally in Bill Dietrich, a German-American college student who has been approached by a German Bund and promised a good job in Germany. When a suspicious Bill reports the incident to the F.B.I., Inspector George A. Briggs tells him to cooperate. After Bill is sent to Germany and enrolled in a specialized spy school, a hit-and-run automobile accident in New York City becomes the catalyst for one of the F.B.I.'s most complicated cases. In the morgue, the attendants discover that although the accident victim has a Spanish passport, he was carrying a notebook filled with German writing. The accident is reported to the F.B.I., which concludes that the man is German spy Franz von Wirt and then decodes a letter he was carrying. The letter, which states that "Mr. Christopher will concentrate on Process 97," alarms Briggs, for Process 97 is the U.S. military's most carefully guarded and important secret: the development of the atomic bomb. Briggs is instructed to make the Christopher case his top priority, and after Bill completes his training in Germany, he returns to New York, where Briggs helps him establish a decoy office. Bill contacts Elsa Gebhardt, a German agent posing as a couterier, at her house on 92nd Street. There, he also meets spies Max Coburg and Conrad Arnulf, and Gestapo agent Johanna Schmidt. Bill pretends to build a shortwave radio station, with which he is supposed to transmit Elsa's information to Hamburg. Actually, Bill's messages are relayed through an F.B.I. radio station, which keeps Briggs abreast of the latest developments. Elsa is suspicious of Bill's credentials, which were altered by the F.B.I. to state that he is authorized to contact all agents known to her, but because she cannot contact Hamburg directly for confirmation, she must trust him. Bill receives information from Col. Hammersohn, a professional spy, but he rebuffs Bill's attempt to learn the identity of Mr. Christopher. Hammersohn introduces Bill to Adolphe Klaen, another member of the spy ring, and Bill witnesses Johanna's ruthlessness when she orchestrates the murder of Klaen's drunken informant. While Bill continues his investigation, the F.B.I. intensifies its efforts after the attack on Pearl Harbor. Many suspected foreign agents are rounded up, although some, such as Elsa and Hammersohn, are allowed to go free in the hope that they will reveal Christopher's identity. Bill is able to obtain an important clue in the form a lipstick-stained cigarette left in Elsa's shop by an acquaintence of Christopher. The F.B.I. uses the clue to track down Luise Vadja, another German agent, who leads the federal agents to Charles Ogden Roper, a scientist working at the Appleton Laboratory, out of which the information is being smuggled. Briggs learns that Roper is a "memory artist" and has been memorizing complicated Process 97 plans and passing them to Christopher. When confronted, the naïve Roper confesses his complicity and says that one of his drop-off points is Adolphe Lange's bookshop. The F.B.I. establishes a survelliance operation opposite the bookstore and identifies Christopher as a man seen at Elsa's building. Meanwhile, Elsa receives a copy of Bill's credentials from Hamburg and thereby learns that the information he gave her was forged. He is brought to her house and is drugged, questioned and beaten by Elsa, Johanna and the others. Briggs and his men surround the house and order the spies to surrender, and when they refuse, they throw tear gas. During the ensuing confusion, Elsa removes her blonde wig and makeup, then dons the men's clothing she wears while enacting the role of Christopher. Due to the tear gas smoke, however, Arnulf does not recognize her, and, believing her to be a strange man, shoots and kills her. The federal agents enter the building and rescue Bill, then round up the rest of the spies. With Christopher's identity revealed and the case closed, Process 97 is safe and the F.B.I. continues its fight against foreign agents.