While investigating a series of mysterious deaths, Sergeant Ben Peterson (James Whitmore) finds a young girl (Sandy Descher) who is unable to speak. As Peterson joins forces with FBI agent Robert Graham (James Arness) and scientist Dr. Harold Medford (Edmund Gwenn), he discovers that all the incidents are due to giant ants that have been mutated by atomic radiation. Peterson and Graham, with the aid of the military, attempt to find the queen ants and destroy the nests before the danger spreads.