The Puppet Masters
Robert A. Heinlein
 "Sam" is an agent in "Section", a United States government intelligence organization. It is so secret it reports only to the President, and is entirely unknown to anyone else. Sam is superbly trained, equipped with a built-in communicator he calls a "skull phone" as well as a number of ways to commit suicide if the need arises. Called in by "the Old Man", his boss and the head of the organization, they go to investigate the report of a flying saucer landing in Grinnell, Iowa after other agents sent earlier fail to report back. With them is another agent named "Mary", a stunningly beautiful redhead. Sam is informed that her life is only slightly less precious than the Old Man's, and that he (Sam) is the most expendable. In Iowa, they discover that the people are being brought under the mental control of repulsive, slug-like creatures that attach to their backs, just below the neck. Detaching one slug from its host, they seal it in a film canister and bring it back to headquarters in Washington, D.C. By the time they get there though, the remains of the slug are a stinking mess, and they are unable to convince the President that there is an invasion. Sam eventually leads a small team back to Iowa. They inadvertently succeed in capturing a live slug, as one agent becomes "hagridden" without them realizing it. However, Mary spots it when he does not react to her allure like a normal male. The agent is unmasked, subdued and confined. The slug escapes by transferring to another person, and eventually to Sam himself. He immediately becomes enslaved and escapes the agency. Meanwhile, the invasion continues to expand. Slugs are shipped through the mail to recruit more humans. Gradually they infect more and more important people, especially the members of exclusive clubs frequented by politicians. Before the Old Man tracks Sam down and captures him, they have infected the Assistant Secretary of the Treasury, whose department controls the United States Secret Service (responsible for the President's personal security); the slugs are a step away from infecting the President himself. By the time Sam recovers in hospital, anyone fully dressed is suspect. The Old Man wants someone to "wear" the slug so it can be interrogated. Sam cannot bear the idea, but when Mary volunteers, he gives in and does it himself. He is completely aware of himself when possessed, but totally committed to the slugs' cause. The slug dies under torture from electric shocks, but in the process Sam learns that they come from Titan, the largest moon of Saturn. Thoroughly disgusted by the treatment he has received, Sam is ready to quit. He is at first furious with Mary for entrapping him, and then with the Old Man, when he learns that his boss tricked Mary into it. At this point, it is revealed that the Old Man is Sam's father. Eventually the politicians realize the danger. From Minnesota to Louisiana, the center of the country has been taken over. On a solo mission to Kansas City, Sam is shocked to discover that the number of slugs is much greater than thought. Instead of taking over key people, they have absorbed the entire human population of the occupied territory. However, he and the Old Man are unable to convince the President to stop a meticulously planned military counter-invasion of the heartland; the entire force is taken over by the enemy. They find that slugs are capable of fissioning in two fairly quickly. The number of infected humans must now be so large that any military action would kill tens of millions. The problem having reached a scale beyond their ability to influence it, Sam and Mary are given leave. By this time, they have fallen in love and get married before going to Sam's bolt-hole in the Adirondack Mountains. Their idyll is interrupted when a slug takes over first his beloved pet cat, then Mary. Sam manages to push Mary onto a hot fire to kill the slug, leaving them both badly burned, but alive and free. Returning to HQ for treatment, they find that a new law requires everyone to be functionally naked to show that a slug is not in control. The law is rigorously enforced not as much by police as by vigilantes, who often shoot first and confirm afterward. Sam begins to believe that the slugs have him marked for repossession. They can communicate by "direct conference", where their hosts sit back to back and the slugs partially merge. A network of such interactions could spread his description rapidly among the invaders, who knew how valuable he is. Some scientists even speculate that the slugs are really just one organism in many bodies. For their part, the slugs drop all pretenses and openly wage war on the states to their east and west. Where human hosts cannot go, they use animals such as dogs, horses and even elephants. Sam and Mary go with the Old Man to investigate a saucer which crash-landed in Mississippi. Inside they encounter the slugs' hosts from Titan, small elf-like creatures, who died when Earth's air entered the ship. There are also tanks containing humans in suspended animation. Mary has a mental breakdown when she enters the ship. It triggers long-suppressed memories from when she was a child in a failed Venus colony which was taken over by slugs. She herself spent years in one of the tanks. Mary caught a disease which killed her slug. They discover that the disease is "Nine-day Fever", which is almost 100% fatal if untreated. However, they find that it kills slugs faster than humans. It might just be possible to spread the fever among the slugs using "direct conference", and then treat as many humans as possible before they die. The Old Man springs a surprise on Sam. He had expected Sam to replace him one day, and Sam would show when he was ready by opposing the Old Man's authority. From now on, Sam is in charge, official titles notwithstanding. Time is short – diseases erupt in the infected areas, as the slugs neglect hygiene and often drive their hosts until they starve. Outbreaks of plague in the Communist countries suggest that they were taken over even before the center of the United States. The counter-attack begins. Releasing animals with infected slugs into enemy territory, they wait for the epidemic to break out. Days pass, and then calls start coming in from desperate people whose slugs have died. Hundreds of thousands of agents, Sam and the Old Man among them, parachute in to treat victims with drug-dispensing guns. Just when the battle seems won, the Old Man is possessed by one of the few healthy slugs and kidnaps Sam, intending to take them both into hiding to regroup for a new invasion. Sam watches in horror as the Old Man's slug begins dividing so he too can be possessed. Despite being tied up, Sam is able to crash their flyer into the sea, killing the slug. In the final section, Sam writes in a journal before embarking with Mary on a spaceship which will take the battle to Titan. The slugs will remain a problem for years to come, having infected too many parts of the Earth to root out easily, but they will never be able to take over.