When a young FBI agent assigned to guard the Lakeview Laboratory of Nuclear Physics, a top secret research facility devoted to the development of the atom bomb, is murdered, Daniel O'Hara, the senior government agent assigned to the project, launches an intense manhunt for his killer. Suspecting that Anton Radchek, an illegal alien, may be involved in the crime, Dan and several other agents trail Radchek to a San Francisco rooming house. Despite constant surveillance by FBI operatives, Radchek is murdered, and Dan's only clue to his killer is a phone call Radchek made to a man named Igor Braun. At San Francisco FBI headquarters, Dan is introduced to Philip Grayson, an emissary from Scotland Yard on a special mission to track down an artist who has been exporting paintings to London, which, under ultra violet light, reveal a secret formula developed at the Lakeview laboratory. When Dan notices that the painting is signed by Igor Braun, the two agents join forces to find the location depicted in the painting. Tracing Braun to an apartment in the city, Dan and Grayson slip in one day while Braun is out to lunch and discover that he is just completing another painting in which is embedded a secret formula. After Braun crates the artwork for shipping, Dan and Grayson learn that it is destined for London. His mission completed in San Francisco, Braun flies to Los Angeles and then drives to an art shop in Lakeview owned by Adolph Mizner, the leader of a communist spy ring. Following Braun, Dan and Grayson then confer with Dr. Frederick Townsend, the director of the Lakeview project. When Townsend identifies the formula as having been developed within the last week, the agents realize one of the five scientists working on the project is a traitor. Through a two-way mirror, the agents observe the scientists--Toni Neva, Ritter Van Stolb, William Forrest, Romer Allen and Townsend--at one of their Friday meetings. The following Monday, Braun sends another painting to London, prompting Dan to surmise that the formula was smuggled out through the plant laundry. Posing as a worker, Grayson infiltrates the laundry and observes Toni drop off a load of clothes from which the clerk extracts a handkerchief, which he then slips into a box of men's shirts. After Krebs, one of the spies, picks up the box, Grayson knocks him unconscious, steals the box and finds a formula inscribed on the handkerchief. Upon regaining consciousness, Krebs returns to Braun's headquarters and recalls that Grayson, a new employee, has just started working at the laundry. Hurrying to Grayson's rooming house, the spies begin to brutalize Grayson and his landlady when Dan arrives. After plucking the handkerchief from Dan's briefcase, Braun orders one of his men to eliminate the agents and leaves the room. Dan and Grayson then overpower their captor, but in the fray, the landlady is killed. Convinced that the handkerchief incriminates Toni and her lover, Van Stolb, the agents question Toni, who protests her innocence. The next day, Van Stolb's body is discovered, a victim of apparent suicide. When a closer investigation reveals that Van Stolb has been murdered, the agents realize that the spies were trying to frame him and decide to flush out the true traitor by forcing him to deliver the formula personally. While following Allen, one of the three remaining scientists, Dan is shot at and forced over a cliff by Krebs. Grayson speeds to rescue Dan from his burning vehicle, and they alert the police about Allen. With the help of the police, the agents locate Allen and Braun closeted in a small house. After a blazing shootout, the agents arrest Allen, the scion of a prominent Boston family, as the traitor. When Allen proclaims his loyalty to his country, Dan forces open his palm, revealing an imprint of the secret formula.