The puppet masters are alien race who came from the Saturn’s moon Titan to Earth to avert humans. Their ships carrying the slugs, small creatures sticking themselves to man’s back and spinal cord to control him, landed mostly in the USA.
A special agent Sam with his agent-partner and future wife Mary is sent to where one of the ships is landed. They learn the slugs can have the power of host. The slugs can think and act the same way as humans do. The purpose of the parasites is to disrupt humans under the mask of bringing them peace.
It turned out that the slugs can spawn very fast. As a result nearly all the US states are contaminated. Many millions are fighting with the free men, those clean of the parasites. We are losing and without a drastic measure we’d become extinct.
Humans found out that they can be stopped and killed by a disease “a nine-day-fever” (reminds me of “The War of The Worlds” by Herbert Wells). They spread that disease through the vast contagious territory and wiped out the slugs.