The Puppet Masters by Robert A Heinlein (1951)
A flying saucer lands in Iowa and almost immediately the locals start acting strangely. Conflicting news reports come in; some saying that the landing is real, others that it's a hoax. US Intelligence sends in a team that quickly discovers that not only is the landing real, but that its passengers are horrible slug-like aliens that latch onto the backs of their human victims and turn them into zombies obedient to the aliens' slightest command. Soon the United States and the world are locked in battle with the invaders for control of the planet and the future of the human race.
Heinlein's 1951 novel was one of Heinlein's first to tackle an adult novel and given the couple of racy scenes of sex and violence that the editors cut out, he made a damn good go of it. Like most science fiction stories, it suffers from the fact that most people only remember it for the "idea" and the problem it poses. What would happen if creatures that could ride people like horses and control them like super Communists invaded the Earth? How would they go about it? How would the Earthmen react? How would each side try to counter the other. A lot of it is fairly logical and the book is probably best remembered for Heinlein's idea that if such a thing happened, mandatory nudity would be implemented. The latter is interesting for three reasons: First, it has story potential; second, it spotlights Heinlein's view that the best way to carry out social engineering is to put a gun to people's heads and three, Heinlein's personal interest in nudism makes it very creepy in retrospect.
The characters, on the other hand are classic Heinlein. The narrator is Sam, an intelligence agent who is the typical Heinlein Competent Man, though the young and still we behind the ears version. It's through him we see the war and what it's like to be a victim of the invaders when he's taken over by one. Then there's the standard Old Man, who is the fully developed Competent Man and acts as Sam's teacher. He's also, we later learn, his father. This provides some personal drama, but not enough to dispel the whiff of the cracker barrel the hangs over all of Heinlein's Old Men,. And finally, Mary, who is the typical Heinlein heroine; Competent, hardly described at all and begging to be made gravid by the Sam. She's also maddening because of Heinlein's insistence on lumbering with pointless things like being able to produce guns out of thin air, making her the centre of a flat romance where Sam literally proposes to her the moment he meets her, and the insufferable tendency of Heinlein to alter her from moment to moment as a convenient plot device.
Part science fiction, part cheap spy thriller, the one place where the novel really falls down is in portraying its world. Though clearly set in the future, Heinlein is never able to sustain any sense of period despite throwing in flying cars, Venus colonies, implanted phones, flying cars and blasters. No matter what prop he rolls out, it still feels solidly like 1951. Perhaps it's because Heinlein makes no bones about the invaders being proxies for the Stalinists who were giving the world so much jip at the time. It sometimes reads like an overblown version of the minutes of the House Unamerican Activities Committee.
Not that this detracts from the fun of the book, and it is fun. Heinlein clearly is aware that he isn't good at portraying mounting dread, so he starts out the plot with a bang and lots of shooting. With a couple of notable lapses, Heinlein keeps the story moving forward with only one really pointless detour when Sam and Mary go on a needless honeymoon and the spots where Heinlein insists on editorialising.
Imitated and baldly ripped-off over the years as well as subjected to a disappointing film treatment in 1994, The Puppet Masters is still a prime example of the Grandmaster of science fiction at the height of his powers.