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After renowned atomic scientist Dr. Karol Noymann dies in an accidental explosion, longtime friend and colleague Dr. Adam Penner resigns from the U.S. Atomic Commission, convinced that continued atomic testing will escalate the dangerous worldwide arms race. The night after Karol's funeral, Adam is stunned to receive a visit from an apparently revived Karol, who reveals that he is an alien using the dead body to give Earth a vital message. The alien declares that his species has grown so alarmed by the human development of space flight and atomic weapons that they are demanding that worldwide atomic testing cease within twenty-four hours or the planet will be destroyed. Adam expresses skepticism over the aliens' ability to conquer the planet, until the extraterrestrial reveals his species is invisible and will inhabit the bodies of dead humans to kill the living. The alien then leaves and later Adam relates the bizarre visit to his adult daughter Phyllis and fellow scientist Dr. John Lamont. Although the couple is incredulous, Adam pleads with John to tell his story to government officials in Washington, D.C.. The story is met with derision and disbelief by officials and the press, frightening Adam into seeking out the alien to plead for an extension. Although doubtful, Phyllis and John accompany Adam to the cemetery and are stunned when Adam's calls bring forth the voice of the alien, who agrees that Adam's efforts to warn the world have not been effective. Vowing to present a warning that cannot be denied, the alien departs. Soon after, a plane crashes in New York and the body of the dead pilot appears at a well attended sporting event to demand the cessation of atomic testing and Earth's surrender to the aliens. The warning is repeated in California with the dead body of a car crash victim and as panicked news reports spread the threat, the United Nations convenes to discuss a course of action. When the U.N. declares they have no intention of giving in to the aliens, the attack on Earth begins with the aliens inhabiting the bodies of the dead to set off numerous calamities that result in fires, floods and devastation. As the destruction spreads, Adam is reappointed to the Atomic Commission and ordered to assist in defense planning. Air Force major Bruce Jay and John escort Adam and Phyllis to the safety of an underground bunker, designed to shield against atomic attacks. At the bunker, Bruce contacts General Stone, who informs them that the American government has gone into hiding for security purposes, as the aliens have refused to negotiate. Bruce reports that they have had to seal themselves in the bunker at the approach of several walking dead, because the invisible aliens give off high amounts of radioactivity. Despite John's misgivings that he and Adam can help, the scientists begin compiling all the known data on the aliens' behavior. Later, Stone notifies them that the aliens are not employing any special powers to wage their assault, but instead are using Earth's own weapons against itself. Adam is invigorated by this information and decides to focus on making the aliens visible. Adam tells Bruce he needs one of the walking dead to experiment with and explains that he has devised a strong, quick-drying acrylic plastic that may be powerful enough to seal the alien inside the body it inhabits. Bruce then dons the bunker's only radiation protection suit and, going outside, uses a spray gun filled with acrylic plastic on a walking dead, but the spray is too slow and the alien flees the body. The men regroup at the bunker and decide to lure a walking dead into a large pit filled with the acrylic liquid. With Bruce serving as bait to bring out more of the walking dead, the plan succeeds and Bruce and John return to the bunker with the alien encased in the acrylic. Placing the body in a special pressure chamber, the scientists shatter the acrylic shell but are disappointed when the alien quickly departs the body and threatens them over the radio system. Adam and John continue trying various tests with light to make the alien visible, without result. Meanwhile, John grows increasingly anxious as numerous walking dead attempt to break into the bunker and, panicking, attempts to escape. Bruce stops John, and when the ensuing fight inadvertently sets off numerous alarms, Adam notices a violent reaction from within the pressure chamber and surmises that sounds waves may affect the aliens. Bruce and Jay help Adam devise a sound gun, which, when turned on the alien, makes it flee its host body, become visible and die. The scientists try to contact Stone with their results, but realize the aliens are jamming the radio signal. Bruce suggests they use the alien's jamming signal to locate their ship and, upon finding the location, Bruce exposes the ship and destroys it with the sound wave gun. Once the world is rid of the invisible invaders, the U.N. reconvenes and, to Adam's relief, the various nations discuss continuing to work in harmony rather than against one another.