Dave Pomeroy, an agent for the National Bureau of Investigation (NBI), is called to investigate the death of a distinguished European nuclear scientist who was admitted to a hospital suffering from severe radiation burns and was murdered there before he could be questioned. Pomeroy discovers that the only person in the United States to have worked with the dead man is Czech scientist Dr. Paul Cerbo. Aided by radiologist Dr. Paula Stevens, Pomeroy learns that Cerbo is in league with two Communist conspirators, August Best and his assistant Myra Pryor, who plan to start World War III by detonating an atom bomb in the heart of Los Angeles. By the time Pomeroy locates the Communist headquarters, the deranged Best has triggered the bomb's reactor so that no one can prevent it from exploding. In an unsuccessful attempt to deactivate the device, Pomeroy is exposed to a fatal overdose of radiation. Warning everyone to stay clear of him, he calls a helicopter, replaces the pilot, flies out over the Pacific with the bomb, and sacrifices himself to save Los Angeles and the world from catastrophe.