After centuries of hushed-up rumours about flying saucers, aliens from Titan invade North America. Their spacecraft land at three different places, the first in Iowa. The slug-like parasites attach themselves to human hosts, controlling their every thought, and through them seize communications, industry and the government.
The narrator, Sam Cavanaugh aka Mr. Nivens aka Elihu, is a member of a special operations team which must stop the invaders and which can trust no one... since the President, Congress, and key leaders may have already been taken over by the Puppet Masters. Also part of the team is his future wife and a weapons expert, Mary aka Allucquere, who has a secretive, unusual past and is not what she seems.
When Sam, himself is possessed by a Master, we learn first-hand what hag-ridden possession is like from the victim's viewpoint, and also glimpses of the slugs's perspective. Afterward, Sam's personal trauma over this ordeal, and then his terror at having to undergo it again is quite real.
Sam's boss, the 'Old Man', knows that the only way to contain the threat is by swift government action, but the President isn't convinced until after an attempt on his life is made. By this time, the aliens already have control of the Midwest, the Red Zone. The President orders Americans to strip to the waist in 'Schedule Bare Back', exposing the shoulder-hugging Masters, and at first this seems to work. The aliens, however, control communications in the Red Zone and simply replace the President's broadcast with old film. They then find other ways of attaching themselves to the body. This calls for the more complete 'Schedule Sun Tan'. Apparently, the abolition of the "body-modesty taboo" is a frequent idea in Heinlein's books, but in this story, at least, there is actually a solid reason provided for it. However, they do concede that they will need to defeat the aliens before the arrival of Winter,
We discover very early on in the book that the 'Old Man' is actually Sam's father. It didn't come as that much of a shock, but once revealed I expected more to be made of it later. There is only a small reference to Sam's mother, and nothing of his childhood.
Once the aliens in Congress are uncovered, a military operation is authorized. Sam is sent first to Missouri to investigate if direct action will defeat the threat. There he finds that almost all the population has been possessed. He fights his way back out, but not in time to prevent the attack, and the military force, overcome by sheer numbers, is quickly absorbed into the collective. 'Schedule Counter Blast' is a military failure. The creatures don't need new spacecraft landings for re-inforcements, they can multiply by fission.
Meanwhile, the Old Man finds information buried in Mary's subconcious mind, from her childhood with the 'Whitmanite' anarchist-pacifist cult (that sounds very like the 'Church of Scientology'.) Her memory of her life on Venus may be the key to our victory over the Titans, via 'Schedule Fever'.
The Puppet Masters was actually first published in 1951, in Horace Gold's 'Galaxy Magazine', very early in the Cold War. The 'mind-controlling invaders' serve as a metaphor for the imagined or real Communists who might infiltrate the US government. The novel has remained continuously in print, easily outliving the Cold War and the menace of "Aunt Sonya". It was even made into a film starring Donald Sutherland as recently as 1994.
The age of the book does mean that it is full of sexist, and some racist, comments by the characters that would never appear today. Some will find the sexism very extreme. Women are always depicted either a "babe" or as "damn fools and children". If a man fails to notice Mary's charms then he automatically must be possessed. In fact, this ability of Mary appears to be her main function in the first half of the book.
