Strange things are happening in Riverdale, Illinois. A huge, seemingly alien structure has been found jutting out of the earth. Sent to investigate the origin of the mysterious object, Senator Walter Powers discovers that parasites from the center of the earth have infiltrated the town, taking control of the authorities and workers, making communication with the outside world impossible, and leaving the responsibility of stopping the invasion up to Powers and a small group of free individuals.
* The producers of this movie were sued by Robert A. Heinlein, who claimed the plot stole several elements of his novel, "The Puppet Masters". The suit was settled out of court.
* Producer Ed Nelson, who also stars, created the parasites himself using little wind-up toys covered with fur from an old coat and pipe cleaners for antennae.
* Some sources have claimed that future exploitation director Jack Hill acted in this film. "Jack Hill" was actually the name being used by veteran actor Cornelius Keefe (who plays Senator Walter K. Powers) for his film credits during the 1950s.
* To save money on his own film, Ed Nelson, who also produced, wears the same coat he used in Night of the Blood Beast (1958).
Factual errors: Leonard Nimoy's name is misspelled in the opening credits as "Leonard Nemoy"