A government agent infiltrates a secret organization, known as The Invisibles, intent on using alien parasites to take over the bodies of important humans.

"You do not know these men. You may have looked at them, but you did not see them. They are newspapers blowing down a gutter on a windy night. For reasons both sociological and psychological these three have never joined or been invited to join society. They have never experienced love or friendship, or formed any lasting or constructive relationship, but today, at last, they will become a part of something. They will belong. They will come a little closer to their unrealistic dreams of power and glory. Today, finally they will join th... I almost said the human race, and that would have been a half-truth, for the race they are joining today is only half human."

Three of society's outcasts — men named Spain, Plannetta, and Castle — are taken to a compound that serves as the headquarters of a subversive secret society known as the Invisibles. In their initial debriefing, Spain immediately recognizes the man speaking to them as the governor of an unnamed state. The "governor" rebuts him, saying that his only role is as ruler of the society of the Invisibles. 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. By doing so, the Invisibles plan to conquer the world. Each of the new recruits will be given a target to infect following his inoculation. Castle's inoculation is unsuccessful, and he ends up deformed, as is one of the aides at the compound.

It turns out that Spain is actually an agent for the GIA, sent to infiltrate the Invisibles. He sneaks out to meet his contact in the woods and states his plan to try to establish a friendship with the desperately lonely Plannetta so he can get Plannetta to call his "kid brother" so that they can meet up later. In reality, the number is that of GIA headquarters. 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. However, the reality is that the Invisibles are onto him. Clarke is already infected, and Spain was sent there so that Clarke could infect him when his inoculation wore off. Spain manages to escape when Clarke says too much and is momentarily tortured by his Invisible. As Spain flees, Clarke's wife is driving up, and she hits him with the car, breaking his leg. Spain blacks out from the pain. When he comes to, Mrs. Clarke is tending to his injury, but he limps away and steals a car before Clarke can find him.

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