The Puppet Masters is Robert Heinlein's 1951 contribution to McCarthyite paranoia and the UFO craze. The novel features American secret agents fighting parasitic invaders from outer space. It was originally serialized in Galaxy Science Fiction (September, October, November 1951).
Reports of an alien spacecraft crash landing near Grinnell Iowa are denied by locals (who have lost their wills to the puppeteers), pretending that the event is a hoax. Investigating Federal agent (code name) Mary Cavanaugh discovers that the locals displaying the hoax fail to register her sexual appeal. "Something was wrong with them. They were dead inside. Harem guards, if you know what I mean." It is soon discovered that slug-like creatures, arriving via flying saucers, are attaching themselves to people's backs, taking control of their victims' nervous systems, and manipulating the victims as puppets. Federal agent (code name) Sam Cavanaugh is taken over by the creatures, but upon liberation and interrogation reveals that they come from Titan. The U.S. President and Congress are ready to accept that the country has been infiltrated and adopt legislation requiring nudity to demonstrate that they are not carrying slugs. Author Heinlein was a nudist.