In California at the U.S. government’s Office of Scientific Investigation, scientists known as A-men investigate the uses of atomic power. One morning, agent Jeffrey Stewart’s pregnant wife Connie drops him off at work, where Jeff’s coworker, agent Dan Forbes, expresses concern because there are high levels of radioactive dust in the air. Nearby at an appliance store, manager Mr. Simon reprimands his employee Albert because their clocks show the wrong time. Albert assures him that he set the clocks correctly the night before, and soon, they notice that all metal implements in the store have become magnetically bonded. When the appliances are physically drawn together by an unidentified force, Simon calls the power department, which then contacts the O.S.I. Jeff and Dan are sent to investigate and their Geiger-Müller counter indicates that there is a high level of radiation emanating from the second floor of the building. Jeff and Dan don protective suits to search the second floor, where they discover a makeshift laboratory and the body of a man who has died of radiation poisoning. At the O.S.I. laboratory, Jeff and Dan test the empty container found in the apartment and then feed the information by phone to a computer known as M.A.N.I.A.C., which stands for “Mathematical Analyzing Numerical Integrator and Computer.” The A-men then make radio broadcasts asking that the public report any unusual activities. Jeff and Dan go to the airport after a taxicab driver calls their secretary Nelly because his automobile has inexplicably shut down, and their supervisor Dr. Allard reports that the airport’s radar is being disrupted by interference. Jeff and Dan warn Kenneth Smith, the airport supervisor, that the cabdriver’s last fare, an elderly man unusually protective of his briefcase, may be carrying a magnetic element that could cause his airplane to crash. The elderly man is Howard Denker, a research physicist at Southwestern University, who grows extremely ill during the flight, but refuses help from the stewardess. The starboard engine fails just as the pilots are ordered to turn back, and the stewardess reports that Denker’s gums are bleeding. Via the radio, Jeff instructs the pilots to place Denker’s briefcase in the rear of the plane. The plane safely returns to the airport where an ailing Denker explains his experiment, in which he had been bombarding serranium with alpha particles, which altered its properties, making it magnetic. Denker did not want to share his discovery, so he rented the apartment above the appliance store to keep his work secret. After his assistant’s death, he decided to return to the university for safety. Denker now warns Jeff that the element requires a constant electric charge or it will expand uncontrollably. After Denker dies, the element is transported to a university cyclotron. Jeff returns home exhausted and concerned about his wife’s health. Although Jeff had hoped to look for a house that day, Allard calls him back to work because the cyclotron has imploded and collapsed, and two men have died. Jeff accompanies Allard to a meeting with university professor Dr. Serny, Capt. Dyer of the Department of Civilian Defense and Col. Willis of the Army. There, Serny explains that everything within 100 yards of the cyclotron has become magnetic, and that although the element is now in a vacuum chamber, it has doubled in size. Serny adds that the element temporarily lost radioactivity during the implosion, which defies known laws of physics. Jeff and Dan perform exhaustive tests to determine the strength of the magnetic field. They then transmit their information to the M.A.N.I.A.C. and, to prevent another implosion, they feed the element with electrons. M.A.N.I.A.C. reports that due to the nature of the element, it will implode every eleven hours and double its mass each time unless it is fed an electrical charge. In order to delay the next implosion, the O.S.I. arranges for the city to divert all power to the lab. In a meeting with the mayor, Jeff hypothesizes that if the element continues to grow, its mass could alter Earth’s orbit. He then recommends that the only way to stop the growth is to overfeed the element with energy until it bursts and creates fusion, thereby becoming two stable elements. Gen. Meehan then arranges with the Canadian government to have the element transported to their top-secret subterranean deltatron in Nova Scotia where they plan to overfeed it with energy. Canadian scientists Dr. Benton and Dr. Cartwright take Jeff and Dan below sea-level to the deltatron. Although Benton protests that the amount of power Jeff intends to generate will destroy the site, he is overruled by his government. Jeff then dismisses the crew for their own safety, and sets the deltatron to maximum power. Benton sabotages the floodgate in an attempt to thwart the experiment, but Jeff cuts the cable, thereby allowing the floodgate doors to close just before a massive explosion bursts through the sea-wall and floods the chamber. Jeff and Dan initially believe they have failed because they see metal objects clinging to the doors, but after the explosions cease, the objects drop to the ground. Later, Connie meets Jeff at the airport and they drive to their new home.