After evidence surfaces of an information leak at a major nuclear lab, FBI Agent Dan O'Hara (Dennis O'Keefe) pairs up with Scotland Yard Detective Philip Grayson (Louis Hayward) to track down those responsible. An investigation of the scientists, including the enigmatic beauty Dr. Toni Neva (Louise Allbritton), leads the agents to a Communist Party spy operation in San Francisco, which they must shut down before sensitive state secrets are handed over to the wrong people.