A government agent infiltrates a secret organization, known as The Invisibles, intent on using alien parasites to take over the bodies of important humans.
Their purpose is to infect key government and business figures with crab-like creatures who attach themselves to people's backs and take over their minds.
Spain's plan hits a hitch when Johnny (his contact) is killed and brought in as a corpse for the trainees to practice on.
Spain is assigned to infect General Hillary Clarke in Washington, D.C., while posing as his faithless wife's chauffeur.
In the nick of time, his fellow GIA agents arrive, shooting both the creature and Plannetta, thus saving Spain's life.
However, Spain can only think of the fact that the Invisible menace is everywhere, although a GIA agent assures him that the threat is over because the agency has captured General Clarke's aide Oliver Fair, who is "cooperating".
"There is an unusual pause in the course of the opening narration which seems to indicate that the Control Voice makes a mistake of appreciation: “Today, finally, they will join the hu...
Neil Hamilton, who plays Gen. Clarke, is perhaps best known for his role as Commissioner Gordon in the 1960s Batman TV series.
Only "Demon With a Glass Hand" and "The Man Who Was Never Born" had received higher fan ratings as of that date.